[LV] Use getBestPlan when interleaving only. (NFCI)

Use the getBestPlan() utility added in b841e2e to also get the
scalar plan when interleaving only.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -10281,7 +10281,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
       InnerLoopUnroller Unroller(L, PSE, LI, DT, TLI, TTI, AC, ORE, IC, &LVL,
                                  &CM, BFI, PSI, Checks);
 
-      VPlan &BestPlan = LVP.getBestPlanFor(VF.Width);
+      VPlan &BestPlan = LVP.getBestPlan();
+      assert(BestPlan.hasScalarVFOnly() &&
+             "VPlan cost model and legacy cost model disagreed");
       LVP.executePlan(VF.Width, IC, BestPlan, Unroller, DT, false);
 
       ORE->emit([&]() {