[VPlan] Consistently use VF.Width to getting plan for main loop VF (NFC)

Cleanup to make things consistent in preparation for
#98821.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -10027,8 +10027,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
         EpilogueVectorizerMainLoop MainILV(L, PSE, LI, DT, TLI, TTI, AC, ORE,
                                            EPI, &LVL, &CM, BFI, PSI, Checks);
 
+        assert(EPI.MainLoopVF == VF.Width && "VFs must match");
         std::unique_ptr<VPlan> BestMainPlan(
-            LVP.getBestPlanFor(EPI.MainLoopVF).duplicate());
+            LVP.getBestPlanFor(VF.Width).duplicate());
         const auto &[ExpandedSCEVs, ReductionResumeValues] = LVP.executePlan(
             EPI.MainLoopVF, EPI.MainLoopUF, *BestMainPlan, MainILV, DT, true);
         ++LoopsVectorized;