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

[Graham Hunter via llvm-commits]

================
@@ -463,14 +462,10 @@ class LoopVectorizationLegality {
     return CountableExitingBlocks;
   }
 
-  /// Returns all the exiting blocks with an uncountable exit.
-  const SmallVector<BasicBlock *, 4> &getUncountableExitingBlocks() const {
-    return UncountableExitingBlocks;
-  }
-
-  /// Returns all the exit blocks from uncountable exiting blocks.
-  SmallVector<BasicBlock *, 4> getUncountableExitBlocks() const {
-    return UncountableExitBlocks;
+  /// Returns all the loop edges that have an uncountable exit.
+  const SmallVector<std::pair<BasicBlock *, BasicBlock *>, 4> &
+  getUncountableEdges() const {
----------------
huntergr-arm wrote:

```suggestion
ArrayRef<std::pair<BasicBlock *, BasicBlock *>> getUncountableEdges() const {
```
nit: I think we can abstract over the SmallVector. ArrayRef only has `const` accessors, so can't be used to change the underlying data.

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