[llvm] [VPlan] Enforce that there is only ever one header mask. NFC (PR #152489)

[Mel Chen via llvm-commits]

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@@ -2242,8 +2246,8 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
     }
   }
 
-  // Try to optimize header mask recipes away to their EVL variants.
-  for (VPValue *HeaderMask : collectAllHeaderMasks(Plan)) {
+  // Try to optimize recipes which use the header mask to their EVL variants.
+  if (VPValue *HeaderMask = findHeaderMask(Plan)) {
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Mel-Chen wrote:

Is there a situation where there is no header mask?
I think it would be better to use an assert here if a header mask always exists when do `addExplicitVectorLength`.

https://github.com/llvm/llvm-project/pull/152489