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

[via llvm-commits]

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@@ -52,6 +52,21 @@ struct VPlanTransforms {
       verifyVPlanIsValid(Plan);
   }
 
+  /// Introduce the top-level VPRegionBlock for the main loop in \p Plan. Coming
+  /// in this function, \p Plan's top-level loop is modeled using a plain CFG.
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ayalz wrote:

```suggestion
  /// into this function, \p Plan's top-level loop is modeled using a plain CFG.
```

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