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

[Luke Lau via llvm-commits]

================
@@ -127,15 +133,33 @@ 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.
+  // Compute the edge masks for all incoming edges to VPBB. Insert after the
+  // block's phis, which will be replaced by blends later.
+  // TODO: Skip creating edge masks for blocks that are control-flow equivalent
+  // to header and have no phis.
   Builder.setInsertPoint(VPBB, VPBB->getFirstNonPhi());
+  for (auto *Predecessor : SetVector<VPBlockBase *>(
+           VPBB->getPredecessors().begin(), VPBB->getPredecessors().end()))
+    createEdgeMask(cast<VPBasicBlock>(Predecessor), VPBB);
+
+  // Reuse the mask of header block if VPBB is control-flow equivalent to
----------------
lukel97 wrote:

```suggestion
  // Reuse the mask of the header if VPBB is post-dominated by it
```

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