[llvm] [VPlan][NFC] Add new getMiddleBlock interface to VPlan (PR #113558)

[Florian Hahn via llvm-commits]

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@@ -3689,6 +3689,14 @@ class VPlan {
   VPBasicBlock *getEntry() { return Entry; }
   const VPBasicBlock *getEntry() const { return Entry; }
 
+  /// Methods to support access to the middle block.
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fhahn wrote:

Might be good to add a bit more context, e.g. something like

```suggestion
  /// Returns the 'middle' block of the plan, that is the block that selects whether to execute the scalar tail loop or the exit block from the loop latch.
```

(or something nicer worded)

https://github.com/llvm/llvm-project/pull/113558