[llvm] [LV] Add -fold-epilogue-tail option for tail-folded epilogue (PR #190697)

[David Sherwood via llvm-commits]

================
@@ -804,6 +812,12 @@ enum EpilogueLowering {
   // Loop hint indicating an epilogue is undesired, apply tail folding.
   CM_EpilogueNotNeededFoldTail,
 
+  // Tail-folded vector epilogue requested; the scalar tail will be folded into
+  // the vectorized epilogue loop if possible, otherwise fall back to an
+  // epilogue. This status is specifically for the CM instance of the
+  // tail-folded epilogue.
+  CM_EpilogueNotNeededFoldEpilogueTail,
----------------
david-arm wrote:

I can't see anything in this PR that uses or sets this new enum so it's not being tested. I think it might be good to either:

1. Remove the enum, or 
2. Add code that tests the enum. Places where the enum is set or used - `isEpilogueAllowed`, `preferTailFoldedLoop`, `setTailFoldingStyle`, `computeMaxVF`, `getEpilogueLowering`, `processLoop`.

For example, in this PR we will still call `getEpilogueLowering` before hitting the report `fold-epilogue-tail is not supported`. In `getEpilogueLowering ` you could return this new enum. Then on line 8388 (processLoop) there is code that checks the enum for low trip counts:

```
      if (SEL != CM_EpilogueNotNeededFoldTail)
        SEL = CM_EpilogueNotAllowedLowTripLoop;
```

In this case without any modifications your new status will get overridden with `CM_EpilogueNotAllowedLowTripLoop` so we won't generate an epilogue. You could use this to write a test for a loop with a low trip count (e.g. 10) while using the new `-tail-folding-policy=fold-epilogue-tail` option. That way we will at least have one test for the new enum? Then on line 8450 instead of testing the new command line option directly you could do:

```
  if (SEL == CM_EpilogueNotNeededFoldEpilogueTail) {
    // TODO: Apply tail-folding on the vectorized epilogue loop.
    LLVM_DEBUG(dbgs() << "LV: epilogue tail-folding is not supported yet\n");
    reportVectorizationInfo("The tail-folding policy fold-epilogue-tail is not "
                            "supported yet, fall back to an epilogue",
                            "UnsupportedTailFoldingPolicy", ORE, L);
  }
```

https://github.com/llvm/llvm-project/pull/190697