[llvm] [VPlan] Use VPInstruction for VPScalarPHIRecipe. (NFCI) (PR #129767)

[via llvm-commits]

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@@ -1074,9 +1074,15 @@ void VPlan::execute(VPTransformState *State) {
         Inc->setOperand(0, State->get(IV->getLastUnrolledPartOperand()));
       continue;
     }
+    if (auto *VPI = dyn_cast<VPInstruction>(&R)) {
+      Value *Phi = State->get(VPI, true);
+      Value *Val = State->get(VPI->getOperand(1), true);
+      cast<PHINode>(Phi)->addIncoming(Val, VectorLatchBB);
+      continue;
+    }
 
     auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
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ayalz wrote:

Suffice to
```suggestion
    auto *PhiR = cast<VPSingleDefRecipe>(&R);
```
so State->get() works for both VPHeaderPHIRecipes and VPInstructions.

https://github.com/llvm/llvm-project/pull/129767