Revert "[mlir][Transforms] Dialect conversion: Simplify handling of dropped arguments (llvm#96207)"

This reverts commit f1e0657.

It breaks SCF conversion, see test case on the PR.

Author: d0k

mlir/docs/DialectConversion.md

@@ -246,13 +246,6 @@ depending on the situation.
 
     -   An argument materialization is used when converting the type of a block
         argument during a [signature conversion](#region-signature-conversion).
-        The new block argument types are specified in a `SignatureConversion`
-        object. An original block argument can be converted into multiple
-        block arguments, which is not supported everywhere in the dialect
-        conversion. (E.g., adaptors support only a single replacement value for
-        each original value.) Therefore, an argument materialization is used to
-        convert potentially multiple new block arguments back into a single SSA
-        value.
 
 *   Source Materialization
 
@@ -266,9 +259,6 @@ depending on the situation.
         *   When a block argument has been converted to a different type, but
             the original argument still has users that will remain live after
             the conversion process has finished.
-        *   When a block argument has been dropped, but the argument still has
-            users that will remain live after the conversion process has
-            finished.
         *   When the result type of an operation has been converted to a
             different type, but the original result still has users that will
             remain live after the conversion process is finished.
@@ -338,40 +328,36 @@ class TypeConverter {
     registerConversion(wrapCallback<T>(std::forward<FnT>(callback)));
   }
 
-  /// All of the following materializations require function objects that are
-  /// convertible to the following form:
-  ///   `std::optional<Value>(OpBuilder &, T, ValueRange, Location)`,
-  /// where `T` is any subclass of `Type`. This function is responsible for
-  /// creating an operation, using the OpBuilder and Location provided, that
-  /// "casts" a range of values into a single value of the given type `T`. It
-  /// must return a Value of the converted type on success, an `std::nullopt` if
-  /// it failed but other materialization can be attempted, and `nullptr` on
-  /// unrecoverable failure. It will only be called for (sub)types of `T`.
-  /// Materialization functions must be provided when a type conversion may
-  /// persist after the conversion has finished.
-
+  /// Register a materialization function, which must be convertible to the
+  /// following form:
+  ///   `Optional<Value> (OpBuilder &, T, ValueRange, Location)`,
+  ///   where `T` is any subclass of `Type`.
+  /// This function is responsible for creating an operation, using the
+  /// OpBuilder and Location provided, that "converts" a range of values into a
+  /// single value of the given type `T`. It must return a Value of the
+  /// converted type on success, an `std::nullopt` if it failed but other
+  /// materialization can be attempted, and `nullptr` on unrecoverable failure.
+  /// It will only be called for (sub)types of `T`.
+  ///
   /// This method registers a materialization that will be called when
-  /// converting (potentially multiple) block arguments that were the result of
-  /// a signature conversion of a single block argument, to a single SSA value
-  /// of a legal type.
+  /// converting an illegal block argument type, to a legal type.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addArgumentMaterialization(FnT &&callback) {
     argumentMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting a legal replacement value back to an illegal source type.
-  /// This is used when some uses of the original, illegal value must persist
-  /// beyond the main conversion.
+  /// converting a legal type to an illegal source type. This is used when
+  /// conversions to an illegal type must persist beyond the main conversion.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addSourceMaterialization(FnT &&callback) {
     sourceMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting an illegal (source) value to a legal (target) type.
+  /// converting type from an illegal, or source, type to a legal type.
   template <typename FnT,
             typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
   void addTargetMaterialization(FnT &&callback) {

mlir/include/mlir/Transforms/DialectConversion.h

@@ -168,8 +168,8 @@ class TypeConverter {
     registerConversion(wrapCallback<T>(std::forward<FnT>(callback)));
   }
 
-  /// All of the following materializations require function objects that are
-  /// convertible to the following form:
+  /// Register a materialization function, which must be convertible to the
+  /// following form:
   ///   `std::optional<Value>(OpBuilder &, T, ValueRange, Location)`,
   /// where `T` is any subclass of `Type`. This function is responsible for
   /// creating an operation, using the OpBuilder and Location provided, that
@@ -179,29 +179,26 @@ class TypeConverter {
   /// unrecoverable failure. It will only be called for (sub)types of `T`.
   /// Materialization functions must be provided when a type conversion may
   /// persist after the conversion has finished.
-
+  ///
   /// This method registers a materialization that will be called when
-  /// converting (potentially multiple) block arguments that were the result of
-  /// a signature conversion of a single block argument, to a single SSA value
-  /// of a legal type.
+  /// converting an illegal block argument type, to a legal type.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addArgumentMaterialization(FnT &&callback) {
     argumentMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting a legal replacement value back to an illegal source type.
-  /// This is used when some uses of the original, illegal value must persist
-  /// beyond the main conversion.
+  /// converting a legal type to an illegal source type. This is used when
+  /// conversions to an illegal type must persist beyond the main conversion.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addSourceMaterialization(FnT &&callback) {
     sourceMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
-  /// converting an illegal (source) value to a legal (target) type.
+  /// converting type from an illegal, or source, type to a legal type.
   template <typename FnT, typename T = typename llvm::function_traits<
                               std::decay_t<FnT>>::template arg_t<1>>
   void addTargetMaterialization(FnT &&callback) {