[llvm] [NFC][LoopVectorize] Cache result of requiresScalarEpilogue (PR #108981)

[Florian Hahn via llvm-commits]

================
@@ -1782,6 +1812,9 @@ class LoopVectorizationCostModel {
 
   /// All element types found in the loop.
   SmallPtrSet<Type *, 16> ElementTypesInLoop;
+
+  /// Keeps track of whether we require a scalar epilogue.
+  std::optional<bool> RequiresScalarEpilogue[2];
----------------
fhahn wrote:

```suggestion
  std::optional<std::pari<bool, bool>> RequiresScalarEpilogue;
```

For consistency, other places use `std::pair`. Also good to document what the first/second elements are for.

https://github.com/llvm/llvm-project/pull/108981