[InstCombine] Fix user iterator invalidation in bitcast of phi transform

This fixes the issue encountered in D71164. Instead of using a
range-based for, manually iterate over the users and advance the
iterator beforehand, so we do not skip any users due to iterator
invalidation.

Differential Revision: https://reviews.llvm.org/D72657

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -2344,7 +2344,10 @@ Instruction *InstCombiner::optimizeBitCastFromPhi(CastInst &CI, PHINode *PN) {
   Instruction *RetVal = nullptr;
   for (auto *OldPN : OldPhiNodes) {
     PHINode *NewPN = NewPNodes[OldPN];
-    for (User *V : OldPN->users()) {
+    for (auto It = OldPN->user_begin(), End = OldPN->user_end(); It != End; ) {
+      User *V = *It;
+      // We may remove this user, advance to avoid iterator invalidation.
+      ++It;
       if (auto *SI = dyn_cast<StoreInst>(V)) {
         assert(SI->isSimple() && SI->getOperand(0) == OldPN);
         Builder.SetInsertPoint(SI);

llvm/test/Transforms/InstCombine/bitcast-phi-uselistorder.ll

@@ -8,16 +8,13 @@ define double @test(i1 %c, i64* %p) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[END:%.*]]
 ; CHECK:       if:
-; CHECK-NEXT:    [[LOAD:%.*]] = load i64, i64* bitcast (double* @Q to i64*), align 8
-; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i64 [[LOAD]] to double
-; CHECK-NEXT:    [[PHITMP:%.*]] = bitcast i64 [[LOAD]] to double
+; CHECK-NEXT:    [[LOAD1:%.*]] = load double, double* @Q, align 8
 ; CHECK-NEXT:    br label [[END]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[TMP1:%.*]] = phi double [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[TMP0]], [[IF]] ]
-; CHECK-NEXT:    [[PHI:%.*]] = phi double [ 0.000000e+00, [[ENTRY]] ], [ [[PHITMP]], [[IF]] ]
-; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i64* [[P:%.*]] to double*
-; CHECK-NEXT:    store double [[TMP1]], double* [[TMP2]], align 8
-; CHECK-NEXT:    ret double [[PHI]]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi double [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[LOAD1]], [[IF]] ]
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i64* [[P:%.*]] to double*
+; CHECK-NEXT:    store double [[TMP0]], double* [[TMP1]], align 8
+; CHECK-NEXT:    ret double [[TMP0]]
 ;
 entry:
   br i1 %c, label %if, label %end