[Reassociate] Make sure EraseInst sets MadeChange

[uabelho]

Summary

EraseInst didn't report that it made IR changes through MadeChange.

It is essential that changes to the IR are reported correctly,
since for example ReassociatePass::run() will indicate that all
analyses are preserved otherwise.
And the CGPassManager determines if the CallGraph is up-to-date
based on status from InstructionCombiningPass::runOnFunction().

[uabelho]

Similar to https://reviews.llvm.org/D34346, but for reassociate instead of instcombine.

[uabelho]

Fixed typos in commit message and test case.

[rnk]

lgtm

[davide]

This change looks fine.

lib/Transforms/Scalar/Reassociate.cpp

if (Instruction *Op = dyn_cast<Instruction>(Ops[i])) {
       // If this is a node in an expression tree, climb to the expression root
       // and add that since that's where optimization actually happens.
       unsigned Opcode = Op->getOpcode();
       while (Op->hasOneUse() && Op->user_back()->getOpcode() == Opcode &&
              Visited.insert(Op).second)
         Op = Op->user_back();
       RedoInsts.insert(Op);
     }
+
+  MadeChange = true;
 }
 
 // Canonicalize expressions of the following form:
 //  x + (-Constant * y) -> x - (Constant * y)
 //  x - (-Constant * y) -> x + (Constant * y)
 Instruction *ReassociatePass::canonicalizeNegConstExpr(Instruction *I) {
   if (!I->hasOneUse() || I->getType()->isVectorTy())
     return nullptr;

test/Transforms/Reassociate/erase_inst_made_change.ll

+; RUN: opt < %s -inline -reassociate -S | FileCheck %s
+
+; This test case exposed a bug in reassociate where EraseInst's
+; removal a dead call wasn't recognized as changing the IR.
+; So when runOnFunction propagated the "made changes" upwards
+; to the CallGraphSCCPass it signalled that no changes had been
+; made, so CallGraphSCCPass assumed that the old CallGraph,
+; as known by that pass manager, still was up-to-date.
+;
+; This was detected as an assert when trying to remove the
+; no longer used function 'bar' (due to incorrect reference
+; count in the CallGraph).
+
+define void @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret void
+entry:
+  call void @bar()
+  ret void
+}
+
+define internal void @bar() noinline nounwind readnone {
+; CHECK-NOT: bar
+entry:
+  ret void
+}
+
+