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

[via llvm-commits]

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@@ -2467,6 +2469,13 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
     return Partial ? Partial->VFScaleFactor : 1;
   }
 
+  /// Set the VFScaleFactor for this reduction phi. Can only be set to a factor
+  /// > 1
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ayalz wrote:

```suggestion
  /// > 1.
```

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