[llvm] [VPlan] Simplify the computation of the block entry mask. (PR #173265)

[Luke Lau via llvm-commits]

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@@ -129,6 +136,15 @@ VPValue *VPPredicator::createEdgeMask(VPBasicBlock *Src, VPBasicBlock *Dst) {
 void VPPredicator::createBlockInMask(VPBasicBlock *VPBB) {
   // Start inserting after the block's phis, which be replaced by blends later.
   Builder.setInsertPoint(VPBB, VPBB->getFirstNonPhi());
+
+  // Reuse the mask of the header if VPBB is post-dominated by the header.
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lukel97 wrote:

Nit
```suggestion
  // Reuse the mask of the header if the VPBB post-dominates the header.
```

https://github.com/llvm/llvm-project/pull/173265