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

[Luke Lau via llvm-commits]

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@@ -2097,6 +2100,7 @@ void VPlanTransforms::addActiveLaneMask(
               [](VPUser *U) { return isa<VPWidenCanonicalIVRecipe>(U); });
   assert(FoundWidenCanonicalIVUser &&
          "Must have widened canonical IV when tail folding!");
+  VPSingleDefRecipe *HeaderMask = findHeaderMask(Plan);
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lukel97 wrote:

We need to hoist the call upwards here because once we create the VPInstruction::ActiveLaneMask the assert is violated

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