[llvm] [LV][NFC] Refactor structures used to maintain uncountable exit info (PR #123219)

[Florian Hahn via llvm-commits]

================
@@ -391,25 +391,22 @@ class LoopVectorizationLegality {
 
   /// Returns true if the loop has an uncountable early exit, i.e. an
   /// uncountable exit that isn't the latch block.
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fhahn wrote:


Might be good to clarify there must be single edge

```suggestion
  /// Returns true if the loop has exactly one uncountable early exit, i.e. an
  /// uncountable exit that isn't the latch block.
```

https://github.com/llvm/llvm-project/pull/123219