[llvm] [VPlan] Split out optimizeEVLMasks. NFC (PR #174925)

[Florian Hahn via llvm-commits]

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@@ -266,6 +266,14 @@ struct VPlanTransforms {
   addExplicitVectorLength(VPlan &Plan,
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fhahn wrote:

migh be a good opporunity to clarfiy the documentation here, perhaps worth mentioning that this replaces the header mask.

>   /// replaces all uses except the canonical IV increment of
> /// VPCanonicalIVPHIRecipe with a VPEVLBasedIVPHIRecipe.

it may also have to introduce a new phi to track the EVL of the previous iteration

https://github.com/llvm/llvm-project/pull/174925