[llvm] [VPlan] Compute blend masks from minimum set of edge masks (PR #184838)

[Andrei Elovikov via llvm-commits]

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@@ -224,10 +232,100 @@ void VPPredicator::createSwitchEdgeMasks(const VPInstruction *SI) {
   setEdgeMask(Src, DefaultDst, DefaultMask);
 }
 
+// Compute the blend masks required for each incoming block for a phi. Do this
+// by traversing the iterated post-dominance frontier for each incoming block
+// until we find an edge that can lead to multiple incoming blocks. The final
+// blend mask is then the disjunction of these edges.
+//
+// In the example below:
+//        a
+//       / \
+//      b   c
+//     / |  |
+//    d  |  |
+//   /|  |  /
+//  / e   f
----------------
eas wrote:

```suggestion
//      b   c
//     / |  |
//    d  g  |
//   /| / \ /
//  / e   f
```

Modify like this to explain why it's safe to ignore b->g ?

https://github.com/llvm/llvm-project/pull/184838