[mlir] Migrate away from PointerUnion::dyn_cast (NFC) (#135770)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we have a call to dyn_cast earlier in the function, implying
that attrOrProp is nonnull.

Author: kazutakahirata

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

@@ -1828,7 +1828,7 @@ void OpEmitter::genPropertiesSupportForBytecode(
           name, tgfmt(namedProperty->prop.getWriteToMlirBytecodeCall(), &fctx));
       continue;
     }
-    const auto *namedAttr = attrOrProp.dyn_cast<const AttributeMetadata *>();
+    const auto *namedAttr = dyn_cast<const AttributeMetadata *>(attrOrProp);
     StringRef name = namedAttr->attrName;
     if (namedAttr->isRequired) {
       readPropertiesMethod << formatv(R"(