[llvm] [VPlan] Remove loop region in optimizeForVFAndUF. (PR #108378)

[via llvm-commits]

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@@ -3846,7 +3844,10 @@ class VPlan {
 
   /// Returns the preheader of the vector loop region.
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ayalz wrote:

```suggestion
  /// Returns the preheader of the vector loop region, if one exists, or null otherwise.
```

https://github.com/llvm/llvm-project/pull/108378