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

[nikic]

Summary

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.

[lebedev.ri]

This seems good to me.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

Instruction *InstCombiner::optimizeBitCastFromPhi(CastInst &CI, PHINode *PN) {
   // If there is a store with type B, change it to type A.
 
 
   // Replace users of BitCast B->A with NewPHI. These will help
   // later to get rid off a closure formed by OldPHI nodes.
   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);
         auto *NewBC =
           cast<BitCastInst>(Builder.CreateBitCast(NewPN, SrcTy));
         SI->setOperand(0, NewBC);
         Worklist.Add(SI);
         assert(hasStoreUsersOnly(*NewBC));

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

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -instcombine < %s | FileCheck %s
 
 @Q = internal unnamed_addr global double 1.000000e+00, align 8
 
 define double @test(i1 %c, i64* %p) {
 ; CHECK-LABEL: @test(
 ; 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
 
 if:
   %load = load i64, i64* bitcast (double* @Q to i64*), align 8
   br label %end
 
 end:
   %phi = phi i64 [ 0, %entry ], [ %load, %if ]
   store i64 %phi, i64* %p, align 8
   %cast = bitcast i64 %phi to double
   ret double %cast
 
   uselistorder i64 %phi, { 1, 0 }
 }