[PatternMatch] Do not match constant expression trunc

Similar to the change previously made for binops, make m_Trunc()
only match instructions, not constant expressions. This is more
likely to cause a crash than do something useful.

Fixes crash reported at:
#92885 (comment)

Author: nikic

llvm/include/llvm/IR/PatternMatch.h

@@ -1948,8 +1948,8 @@ m_IntToPtr(const OpTy &Op) {
 
 /// Matches Trunc.
 template <typename OpTy>
-inline CastOperator_match<OpTy, Instruction::Trunc> m_Trunc(const OpTy &Op) {
-  return CastOperator_match<OpTy, Instruction::Trunc>(Op);
+inline CastInst_match<OpTy, TruncInst> m_Trunc(const OpTy &Op) {
+  return CastInst_match<OpTy, TruncInst>(Op);
 }
 
 /// Matches trunc nuw.
@@ -1967,7 +1967,7 @@ m_NSWTrunc(const OpTy &Op) {
 }
 
 template <typename OpTy>
-inline match_combine_or<CastOperator_match<OpTy, Instruction::Trunc>, OpTy>
+inline match_combine_or<CastInst_match<OpTy, TruncInst>, OpTy>
 m_TruncOrSelf(const OpTy &Op) {
   return m_CombineOr(m_Trunc(Op), Op);
 }

llvm/test/Transforms/InstCombine/icmp-of-trunc-ext.ll

@@ -642,3 +642,17 @@ define i1 @test_slt_nuw(i32 %x, i16 %y) {
   %cond = icmp slt i8 %conv1, %conv2
   ret i1 %cond
 }
+
+@foo = external global i8
+@bar = external global i8
+
+define i1 @constexpr_trunc() {
+; CHECK-LABEL: @constexpr_trunc(
+; CHECK-NEXT:    [[CMP5:%.*]] = icmp ne i16 trunc (i64 sub (i64 sub (i64 ptrtoint (ptr @bar to i64), i64 ptrtoint (ptr @foo to i64)), i64 8) to i16), trunc (i64 sub (i64 sub (i64 0, i64 ptrtoint (ptr @foo to i64)), i64 8) to i16)
+; CHECK-NEXT:    ret i1 [[CMP5]]
+;
+  %conv = zext i16 trunc (i64 sub (i64 sub nuw nsw (i64 ptrtoint (ptr @bar to i64), i64 ptrtoint (ptr @foo to i64)), i64 8) to i16) to i32
+  %conv1 = zext i16 trunc (i64 sub (i64 sub nuw nsw (i64 0, i64 ptrtoint (ptr @foo to i64)), i64 8) to i16) to i32
+  %cmp5 = icmp ne i32 %conv, %conv1
+  ret i1 %cmp5
+}