[VPlan] Replace scalar preheader with VPIRBB at single place (NFC).

Replace the scalar preheader VPBB with an VPIRBB wrapping the IR basic
block created by createVectorizedLoopSkeleton.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2492,7 +2492,6 @@ BasicBlock *InnerLoopVectorizer::createVectorizedLoopSkeleton() {
   // to the scalar loop.
   emitIterationCountCheck(LoopScalarPreHeader);
 
-  replaceVPBBWithIRVPBB(Plan.getScalarPreheader(), LoopScalarPreHeader);
   replaceVPBBWithIRVPBB(VectorPHVPBB, LoopVectorPreHeader);
   return LoopVectorPreHeader;
 }
@@ -7335,6 +7334,8 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   BasicBlock *EntryBB =
       cast<VPIRBasicBlock>(BestVPlan.getEntry())->getIRBasicBlock();
   State.CFG.PrevBB = ILV.createVectorizedLoopSkeleton();
+  replaceVPBBWithIRVPBB(BestVPlan.getScalarPreheader(),
+                        State.CFG.PrevBB->getSingleSuccessor());
   VPlanTransforms::removeDeadRecipes(BestVPlan);
 
   assert(verifyVPlanIsValid(BestVPlan, true /*VerifyLate*/) &&
@@ -7466,7 +7467,6 @@ BasicBlock *EpilogueVectorizerMainLoop::createEpilogueVectorizedLoopSkeleton() {
   EPI.MainLoopIterationCountCheck =
       emitIterationCountCheck(LoopScalarPreHeader, false);
 
-  replaceVPBBWithIRVPBB(Plan.getScalarPreheader(), LoopScalarPreHeader);
   return LoopVectorPreHeader;
 }
 
@@ -7601,7 +7601,6 @@ EpilogueVectorizerEpilogueLoop::createEpilogueVectorizedLoopSkeleton() {
       Phi->removeIncomingValue(MemCheckBlock);
   }
 
-  replaceVPBBWithIRVPBB(Plan.getScalarPreheader(), LoopScalarPreHeader);
   return LoopVectorPreHeader;
 }