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

[via llvm-commits]

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@@ -9971,8 +9968,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
       InnerLoopUnroller Unroller(L, PSE, LI, DT, TLI, TTI, AC, ORE, IC, &LVL,
                                  &CM, BFI, PSI, Checks);
 
-      VPlan &BestPlan = LVP.getBestPlan();
-      assert(BestPlan.hasScalarVFOnly() &&
+      ElementCount BestVF = LVP.getBestVF();
+      VPlan &BestPlan = LVP.getBestPlanFor(BestVF);
+      assert(BestVF.isScalar() &&
              "VPlan cost model and legacy cost model disagreed");
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ayalz wrote:

```suggestion
      assert(BestVF.isScalar() &&
             "VPlan cost model and legacy cost model disagreed");
      VPlan &BestPlan = LVP.getBestPlanFor(BestVF);
```

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