[llvm] [VPlan] Use ResumePhi to create reduction resume phis. (PR #110004)

[via llvm-commits]

================
@@ -7561,67 +7561,53 @@ static void addRuntimeUnrollDisableMetaData(Loop *L) {
   }
 }
 
-// Check if \p RedResult is a ComputeReductionResult instruction, and if it is
-// create a merge phi node for it.
-static void createAndCollectMergePhiForReduction(
-    VPInstruction *RedResult,
-    VPTransformState &State, Loop *OrigLoop, BasicBlock *LoopMiddleBlock,
-    bool VectorizingEpilogue) {
+// If \p R is a ComputeReductionResult when vectorizing the epilog loop,
+// update the reduction's scalar PHI node by adding the incoming value from the
+// main vector loop.
+static void updateMergePhiForReductionForEpilogueVectorization(
+    VPRecipeBase *R, VPTransformState &State, Loop *OrigLoop,
+    BasicBlock *LoopMiddleBlock) {
+  auto *RedResult = dyn_cast<VPInstruction>(R);
   if (!RedResult ||
       RedResult->getOpcode() != VPInstruction::ComputeReductionResult)
     return;
 
   auto *PhiR = cast<VPReductionPHIRecipe>(RedResult->getOperand(0));
   const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
-
-  Value *FinalValue = State.get(RedResult, VPLane(VPLane::getFirstLane()));
-  auto *ResumePhi =
-      dyn_cast<PHINode>(PhiR->getStartValue()->getUnderlyingValue());
-  if (VectorizingEpilogue && RecurrenceDescriptor::isAnyOfRecurrenceKind(
-                                 RdxDesc.getRecurrenceKind())) {
+  PHINode *MainResumePhi;
+  if (RecurrenceDescriptor::isAnyOfRecurrenceKind(
+          RdxDesc.getRecurrenceKind())) {
     auto *Cmp = cast<ICmpInst>(PhiR->getStartValue()->getUnderlyingValue());
     assert(Cmp->getPredicate() == CmpInst::ICMP_NE);
     assert(Cmp->getOperand(1) == RdxDesc.getRecurrenceStartValue());
----------------
ayalz wrote:

```suggestion
  Value *MainResumeValue = EpiRedHeaderPhiR->getStartValue()->getUnderlyingValue();
  if (RecurrenceDescriptor::isAnyOfRecurrenceKind(
          RdxDesc.getRecurrenceKind())) {
    auto *Cmp = cast<ICmpInst>(MainResumeValue);
    assert(Cmp->getPredicate() == CmpInst::ICMP_NE && "AnyOf expected to start with ICMP_NE");
    assert(Cmp->getOperand(1) == RdxDesc.getRecurrenceStartValue() && "AnyOf expected to start by comparing main resume value to original start value");
    MainResumeValue = Cmp->getOperand(0);
  }
  auto *MainResumePhi = cast<PHINode>(MainResumeValue);
```

https://github.com/llvm/llvm-project/pull/110004