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

[via llvm-commits]

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@@ -9985,15 +9983,20 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     } else {
       // If we decided that it is *legal* to vectorize the loop, then do it.
 
+      ElementCount BestVF = LVP.getBestVF();
+      VPlan &BestPlan = LVP.getBestPlanFor(BestVF);
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ayalz wrote:

```suggestion
```
better set closer to where its used below.

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