[llvm] [VPlan] Add new VPInstruction ocpode for header mask. (PR #89603)

[via llvm-commits]

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@@ -1306,6 +1253,10 @@ void VPlanTransforms::addActiveLaneMask(
 /// ...
 ///
 void VPlanTransforms::addExplicitVectorLength(VPlan &Plan) {
+  VPValue *HeaderMask = getHeaderMask(Plan);
+  if (!HeaderMask)
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ayalz wrote:

If addExplicitVectorLength() is called header mask is expected to exist - assert instead of early exit?

https://github.com/llvm/llvm-project/pull/89603