[llvm] [VPlan] Explicitly reassociate header mask in logical and (PR #180898)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1614,6 +1605,27 @@ void VPlanTransforms::simplifyRecipes(VPlan &Plan) {
   }
 }
 
+/// Reassociate (headermask && x) && y -> headermask && (x && y) to allow the
+/// header mask to be simplified further, e.g. in optimizeEVLMasks.
+static void reassociateHeaderMask(VPlan &Plan) {
+  VPValue *HeaderMask = vputils::findHeaderMask(Plan);
+  if (!HeaderMask)
+    return;
+  ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<VPBlockBase *>> RPOT(
+      Plan.getEntry());
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artagnon wrote:

```suggestion
      Plan.getVectorLoopRegion());
```

?

https://github.com/llvm/llvm-project/pull/180898