[llvm] [VPlan] Update getBestPlan to return VF, use also for epilogue vec. (PR #98821)

[via llvm-commits]

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@@ -7496,8 +7496,7 @@ VPlan &LoopVectorizationPlanner::getBestPlan() const {
       }
     }
   }
-  BestPlan->setVF(BestFactor.Width);
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ayalz wrote:

Agreed it is better to avoid restricting BestPlan's VF's to BestFactor.Width alone, thereby discarding all other associated VF's, one of which may be reuseful for subsequent epilog vectorization. This was the original intent of LVP's getBestPlan(). Cloning could be employed to retain the other VF's, but may be redundant - in case they are not needed.

https://github.com/llvm/llvm-project/pull/98821