[llvm] [LV] Use wide lane masks as the canonical form when tail-folding & interleaving (PR #209484)

[Kerry McLaughlin via llvm-commits]

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@@ -3652,12 +3638,10 @@ LoopVectorizationPlanner::selectInterleaveCount(VPlan &Plan, ElementCount VF,
   // 3. We don't interleave if we think that we will spill registers to memory
   // due to the increased register pressure.
 
-  // Only interleave tail-folded loops if wide lane masks are requested, as the
-  // overhead of multiple instructions to calculate the predicate is likely
-  // not beneficial. If an epilogue is not allowed for any other reason,
-  // do not interleave.
-  if (!CM.isEpilogueAllowed() &&
-      !(CM.preferTailFoldedLoop() && CM.useWideActiveLaneMask()))
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kmclaughlin-arm wrote:

This was intended to revert `selectInterleaveCount` back to the original code before https://github.com/llvm/llvm-project/pull/163387, however `CM.preferTailFoldedLoop()` should also have been removed. I've removed this in the latest commit, which prevents any tail-folded loop from being interleaved without forcing as it was originally.

https://github.com/llvm/llvm-project/pull/209484