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

[Alexey Bataev via llvm-commits]

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@@ -2069,7 +2069,7 @@ static SmallVector<VPValue *> collectAllHeaderMasks(VPlan &Plan) {
 
   // Walk users of wide canonical IVs and collect to all compares of the form
   // (ICMP_ULE, WideCanonicalIV, backedge-taken-count).
-  SmallVector<VPValue *> HeaderMasks;
+  SmallVector<VPSingleDefRecipe *> HeaderMasks;
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alexey-bataev wrote:

Why need an array here? Should be anough single pointer and an assert in the loop

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