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

[via llvm-commits]

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@@ -595,6 +568,19 @@ static void legalizeAndOptimizeInductions(VPlan &Plan, ScalarEvolution &SE) {
     auto *WideIV = dyn_cast<VPWidenIntOrFpInductionRecipe>(&Phi);
     if (!WideIV)
       continue;
+
+    // If there is a header mask, check if WideIV is canonical IV with other
+    // wide users. If that is the case, use it as HeaderMask's operand, so it
+    // can be used when lowering the recipe.
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ayalz wrote:

Is this part needed to retain current behavior, and deserves a TODO to simplify it?

Will this be simpler if the abstract HeaderMask is a "live-in" VPValue of loop region, free of any operands, i.e., using neither the scalar canonical IV nor a wide canonical/WideIV.

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