[IndVarSimplify] Fix Modified status for removal of overflow intrinsics

When removing an overflow intrinsic the Changed status in SimplifyIndvar
was not set, leading to the IndVarSimplify pass returning an incorrect
status.

This was caught using the check introduced by D80916.

As pointed out in the code review, a similar bug may exist for
eliminateTrunc().

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D85971

Author: dstenb

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -477,6 +477,7 @@ bool SimplifyIndvar::eliminateOverflowIntrinsic(WithOverflowInst *WO) {
   if (WO->use_empty())
     WO->eraseFromParent();
 
+  Changed = true;
   return true;
 }
 

llvm/test/Transforms/IndVarSimplify/eliminate-overflow-modified.ll

@@ -0,0 +1,46 @@
+; RUN: opt < %s -indvars -S -o - | FileCheck %s
+
+; When eliminating the overflow intrinsic the indvars pass would incorrectly
+; return a false Modified status. This was caught by the pass return
+; status check that is hidden under EXPENSIVE_CHECKS.
+
+; CHECK-LABEL: for.body:
+; CHECK-NEXT: %0 = phi i16 [ %1, %for.body ], [ undef, %for.body.preheader ]
+; CHECK-NEXT: %1 = add nsw i16 %0, -1
+; CHECK-NEXT: %cmp = icmp sgt i16 %1, 0
+; CHECK-NEXT:  call void @llvm.assume(i1 %cmp)
+
+; Function Attrs: nounwind
+define void @foo() #0 {
+entry:
+  %cmp1 = icmp sgt i16 undef, 0
+  br i1 %cmp1, label %for.body.preheader, label %for.end
+
+for.body.preheader:                               ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.body
+  %0 = phi i16 [ %2, %for.body ], [ undef, %for.body.preheader ]
+  %1 = call { i16, i1 } @llvm.sadd.with.overflow.i16(i16 %0, i16 -1)
+  %2 = extractvalue { i16, i1 } %1, 0
+  %cmp = icmp sgt i16 %2, 0
+  call void @llvm.assume(i1 %cmp)
+  br label %for.body
+
+for.end:                                          ; preds = %entry
+  ret void
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare { i16, i1 } @llvm.sadd.with.overflow.i16(i16, i16) #1
+
+; Function Attrs: nounwind willreturn
+declare void @llvm.assume(i1) #2
+
+attributes #0 = { nounwind }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { nounwind willreturn }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 12.0.0"}