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

[via llvm-commits]

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@@ -8206,56 +8160,12 @@ bool VPRecipeBuilder::getScaledReductions(
 
 VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
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ayalz wrote:

Worth being more specific, as in
```suggestion
VPRecipeBase *VPRecipeBuilder::tryToCreateWidenNonPhiRecipe(VPSingleDefRecipe *R,
```
?

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