[clang] [llvm] [LV][NFC] Rename PreferPredicateOverEpilogue to PreferTailFolding (PR #191803)

[David Sherwood via cfe-commits]

================
@@ -205,36 +205,34 @@ static cl::opt<bool> ForceTargetSupportsMaskedMemoryOps(
     cl::desc("Assume the target supports masked memory operations (used for "
              "testing)."));
 
-// Option prefer-predicate-over-epilogue indicates that an epilogue is undesired,
-// that predication is preferred, and this lists all options. I.e., the
-// vectorizer will try to fold the tail-loop (epilogue) into the vector body
-// and predicate the instructions accordingly. If tail-folding fails, there are
-// different fallback strategies depending on these values:
-namespace PreferPredicateTy {
-  enum Option {
-    ScalarEpilogue = 0,
-    PredicateElseScalarEpilogue,
-    PredicateOrDontVectorize
-  };
-} // namespace PreferPredicateTy
+// Option prefer-tail-folding indicates that an epilogue is undesired, that
+// tail folding is preferred, and this lists all options. I.e., the vectorizer
+// will try to fold the tail-loop (epilogue) into the vector body and the
+// instructions accordingly. If tail-folding fails, there are different fallback
+// strategies depending on these values:
+namespace PreferTailFoldingTy {
+enum Option {
+  PreferEpilogue = 0,
----------------
david-arm wrote:

Given that `Option` lies within the namespace `TailFoldingPolicyTy` it should also be safe to use `None` instead of `PreferEpilogue` if that's simpler? Also, what do you think about being a bit stronger in the names, i.e.

```
namespace TailFoldingPolicyTy {
enum Option {
  None = 0,
  PreferFoldTail,
  MustFoldTail
};
} // namespace TailFoldingPolicyTy
```

? 'Prefer' here gives a bit more flexibility in the interpretation, since we may still not decide to create an epilogue even if we don't tail-fold, for example we might know the trip count will always be a multiple of any chosen VF. `MustFoldTail` implies no other style of vectorisation is permitted and if the legality checks fail we cannot vectorise.

It also has the advantage of shortening the options. :)

https://github.com/llvm/llvm-project/pull/191803