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

[Florian Hahn via llvm-commits]

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@@ -10336,6 +10335,19 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     } else {
       // If we decided that it is *legal* to vectorize the loop, then do it.
 
+      ElementCount Width = VF.Width;
+      std::optional<VPlan *> VPlanFromVPCost;
+      if (!UseLegacyCostModel) {
+        const auto &[VPWidth, Plan] = LVP.getBestPlan();
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fhahn wrote:

I removed the temporary UseLegacyCostModel option now that we branched for 19.x, so this is now simpler again 

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