[llvm] [VPlan] Fix LastActiveLane assertion on scalar VF (PR #167897)

[Luke Lau via llvm-commits]

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@@ -66,6 +66,17 @@ bool vputils::isHeaderMask(const VPValue *V, const VPlan &Plan) {
                       m_One(), m_Specific(&Plan.getVF()))) ||
             IsWideCanonicalIV(A));
 
+  // For scalar plans, the header mask uses the scalar steps.
+  if (match(V,
+            m_ICmp(m_ScalarIVSteps(
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lukel97 wrote:

Good point, I've added a helper in efca5e5a1abe71378f0864896a59ab62c2673bb7

https://github.com/llvm/llvm-project/pull/167897