[InstSimplify] Report "Changed" also when only deleting dead instructions

[bjope]

Summary

Make sure that we report that changes has been made
by InstSimplify also in situations when only trivially
dead instructions has been removed. If for example a call
is removed the call graph must be updated.

Bug seem to have been introduced by llvm-svn r367173
(commit 02b9e45a7e4b81), since the code in question
was rewritten in that commit.

[spatel]

LGTM.

I stepped through this example, and wouldInstructionBeTriviallyDead() returns true because the call has neither uses nor side effects. So we mark it as dead, and then it gets deleted. I'm not sure if that explains the other bug reports mentioned in D65336 though.

llvm/lib/Transforms/Scalar/InstSimplifyPass.cpp

for (BasicBlock &BB : F) {
         // simplify all instructions. On later iterations, ToSimplify is not
         // empty and we only bother simplifying instructions that are in it.
         if (!ToSimplify->empty() && !ToSimplify->count(&I))
           continue;
 
         // Don't waste time simplifying dead/unused instructions.
         if (isInstructionTriviallyDead(&I)) {
           DeadInstsInBB.push_back(&I);
+          Changed = true;
         } else if (!I.use_empty()) {
           if (Value *V = SimplifyInstruction(&I, SQ, ORE)) {
             // Mark all uses for resimplification next time round the loop.
             for (User *U : I.users())
               Next->insert(cast<Instruction>(U));
             I.replaceAllUsesWith(V);
             ++NumSimplified;
             Changed = true;

llvm/test/Transforms/InstSimplify/remove-dead-call.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instsimplify -S -o /dev/null -debug-pass=Details 2>&1 | FileCheck --check-prefix DETAILS %s
+; RUN: opt < %s -instsimplify -S -o - | FileCheck %s
+
+; Verify that InstSimplifyLegacyPass notifies the pass manager about changes
+; being made (when a call is removed CGSCC must be updated).
+;
+; DETAILS: Made Modification 'Remove redundant instructions' on Function 'main'
+
+define internal void @func_1(i64* nocapture readnone %0) #0 {
+; CHECK-LABEL: @func_1(
+; CHECK-NEXT:    unreachable
+;
+  unreachable
+}
+
+define i16 @main(i16 %0, i16** nocapture readnone %1) #1 {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  bb1:
+; CHECK-NEXT:    unreachable
+;
+bb1:
+  call void @func_1(i64* undef)
+  unreachable
+}
+
+attributes #0 = { noinline norecurse nounwind readnone }
+attributes #1 = { norecurse nounwind readnone }