[LV] Pass resume phi to fixReductionScalarResumeWhenVectorizing (NFC).

fixReductionScalarResumeWhenVectorizingEpilog updates the resume phis in
the scalar preheader. Instead of looking at all recipes in the middle
block and finding their resume-phi users we can iterate over all resume
phis in the scalar preheader directly.

This slightly simplifies the code and removes the need to look for the
resume phi.

Also slightly simplifies #141860.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7218,12 +7218,15 @@ static Value *getStartValueFromReductionResult(VPInstruction *RdxResult) {
   return StartVPV->getLiveInIRValue();
 }
 
-// If \p R is a Compute{Reduction,AnyOf,FindLastIV}Result when vectorizing the
+// If \p EpiResumePhiR is resume VPPhi for a reduction when vectorizing the
 // epilog loop, fix the reduction's scalar PHI node by adding the incoming value
 // from the main vector loop.
 static void fixReductionScalarResumeWhenVectorizingEpilog(
-    VPRecipeBase *R, VPTransformState &State, BasicBlock *BypassBlock) {
-  auto *EpiRedResult = dyn_cast<VPInstruction>(R);
+    VPPhi *EpiResumePhiR, VPTransformState &State, BasicBlock *BypassBlock) {
+  // Get the VPInstruction computing the reduction result in the middle block.
+  // The first operand may not be from the middle block if it is not connected
+  // to the scalar preheader. In that case, there's nothing to fix.
+  auto *EpiRedResult = dyn_cast<VPInstruction>(EpiResumePhiR->getOperand(0));
   if (!EpiRedResult ||
       (EpiRedResult->getOpcode() != VPInstruction::ComputeAnyOfResult &&
        EpiRedResult->getOpcode() != VPInstruction::ComputeReductionResult &&
@@ -7274,12 +7277,7 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
   // When fixing reductions in the epilogue loop we should already have
   // created a bc.merge.rdx Phi after the main vector body. Ensure that we carry
   // over the incoming values correctly.
-  using namespace VPlanPatternMatch;
-  assert(count_if(EpiRedResult->users(), IsaPred<VPPhi>) == 1 &&
-         "ResumePhi must have a single user");
-  auto *EpiResumePhiVPI =
-      cast<VPInstruction>(*find_if(EpiRedResult->users(), IsaPred<VPPhi>));
-  auto *EpiResumePhi = cast<PHINode>(State.get(EpiResumePhiVPI, true));
+  auto *EpiResumePhi = cast<PHINode>(State.get(EpiResumePhiR, true));
   EpiResumePhi->setIncomingValueForBlock(
       BypassBlock, MainResumePhi->getIncomingValueForBlock(BypassBlock));
 }
@@ -7388,17 +7386,13 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
       }
     }
     VPBasicBlock *ScalarPH = BestVPlan.getScalarPreheader();
-    ArrayRef<VPBlockBase *> ScalarPreds = ScalarPH->getPredecessors();
-    if (!ScalarPreds.empty()) {
+    if (ScalarPH->getNumPredecessors() > 0) {
       // If ScalarPH has predecessors, we may need to update its reduction
-      // resume values. If there is a middle block, it must be the first
-      // predecessor. Note that the first predecessor may not be the middle
-      // block, if the middle block doesn't branch to the scalar preheader. In
-      // that case, fixReductionScalarResumeWhenVectorizingEpilog will be a
-      // no-op.
-      auto *MiddleVPBB = cast<VPBasicBlock>(ScalarPreds[0]);
-      for (VPRecipeBase &R : *MiddleVPBB)
-        fixReductionScalarResumeWhenVectorizingEpilog(&R, State, BypassBlock);
+      // resume values.
+      for (VPRecipeBase &R : ScalarPH->phis()) {
+        fixReductionScalarResumeWhenVectorizingEpilog(cast<VPPhi>(&R), State,
+                                                      BypassBlock);
+      }
     }
   }