[llvm] [VPlan] Create header phis once, after constructing VPlan0 (NFC). (PR #168291)

[via llvm-commits]

================
@@ -8357,6 +8265,9 @@ VPRecipeBuilder::tryToCreatePartialReduction(VPInstruction *Reduction,
        cast<VPReductionRecipe>(BinOpRecipe)->isPartialReduction()))
     std::swap(BinOp, Accumulator);
 
+  if (auto *RedPhiR = dyn_cast<VPReductionPHIRecipe>(Accumulator))
+    RedPhiR->setVFScaleFactor(ScaleFactor);
----------------
ayalz wrote:

ReductionPHIRecipe currently gets its Style/ScaleFactor set when constructed:
```
      // If the PHI is used by a partial reduction, set the scale factor.
      bool UseInLoopReduction = CM.isInLoopReduction(Phi);
      bool UseOrderedReductions = CM.useOrderedReductions(RdxDesc);
      unsigned ScaleFactor =
          getScalingForReduction(RdxDesc.getLoopExitInstr()).value_or(1);

      PhiRecipe = new VPReductionPHIRecipe(
          Phi, RdxDesc.getRecurrenceKind(), *StartV,
          getReductionStyle(UseInLoopReduction, UseOrderedReductions,
                            ScaleFactor),
          RdxDesc.hasUsesOutsideReductionChain());
```

Setting ScaleFactor here is needed now as noted below, but could this change be applied independently?

https://github.com/llvm/llvm-project/pull/168291