[llvm] [VPlan] Replace PhiR operand of ComputeRdxResult with VPIRFlags. (PR #174026)

[via llvm-commits]

================
@@ -7305,8 +7305,20 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
        EpiRedResult->getOpcode() != VPInstruction::ComputeFindIVResult))
     return;
 
-  auto *EpiRedHeaderPhi =
-      cast<VPReductionPHIRecipe>(EpiRedResult->getOperand(0));
+  VPReductionPHIRecipe *EpiRedHeaderPhi;
+  if (EpiRedResult->getOpcode() == VPInstruction::ComputeReductionResult) {
+    // Find the reduction phi by looking at the other user of operand 0.
+    VPValue *Op = EpiRedResult->getOperand(0);
+    auto *OtherUser = *find_if(
+        Op->users(), [EpiRedResult](VPUser *U) { return U != EpiRedResult; });
+    if (auto *Phi = dyn_cast<VPReductionPHIRecipe>(OtherUser))
+      EpiRedHeaderPhi = Phi;
+    else // For truncated reductions, look through the cast.
----------------
ayalz wrote:

nit: can fold into the declaration of EpiRedHeaderPhi above:
```
    VPReductionPHIRecipe *EpiRedHeaderPhi = dyn_cast<VPReductionPHIRecipe>(OtherUser));
```
or into
```
    if (!EpiRedHeaderPhi = dyn_cast<VPReductionPHIRecipe>(OtherUser))
```

https://github.com/llvm/llvm-project/pull/174026