[llvm] [VPlan] Refactor VPlan creation, add transform introducing region (NFC). (PR #128419)

[via llvm-commits]

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@@ -180,7 +180,7 @@ VPBasicBlock *PlainCFGBuilder::getOrCreateVPBB(BasicBlock *BB) {
 
   // Get or create a region for the loop containing BB.
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ayalz wrote:

```suggestion
  // Get or create a region for the loop containing BB, except for the top region of TheLoop which is created later.
```

https://github.com/llvm/llvm-project/pull/128419