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

[via llvm-commits]

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@@ -365,8 +365,8 @@ class LoopVectorizationPlanner {
   /// Return the best VPlan for \p VF.
   VPlan &getBestPlanFor(ElementCount VF) const;
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ayalz wrote:

```suggestion
  /// Return the VPlan for \p VF.
  VPlan &getPlanFor(ElementCount VF) const;
```
would be more accurate, independent of this patch.

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