[VPlan][NFC] Add new getMiddleBlock interface to VPlan

This work is in preparation for PRs llvm#112138 and llvm#88385 where
the middle block is not guaranteed to be the immediate successor
to the region block. I've simply add new getMiddleBlock()
interfaces to VPlan that for now just return

cast<VPBasicBlock>(VectorRegion->getSingleSuccessor())

Once PR llvm#112138 lands we'll need to do more work to discover
the middle block.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7692,8 +7692,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   BestVPlan.execute(&State);
 
   // 2.5 Collect reduction resume values.
-  auto *ExitVPBB =
-      cast<VPBasicBlock>(BestVPlan.getVectorLoopRegion()->getSingleSuccessor());
+  auto *ExitVPBB = BestVPlan.getMiddleBlock();
   for (VPRecipeBase &R : *ExitVPBB) {
     createAndCollectMergePhiForReduction(
         dyn_cast<VPInstruction>(&R), State, OrigLoop,
@@ -8778,8 +8777,7 @@ static void addCanonicalIVRecipes(VPlan &Plan, Type *IdxTy, bool HasNUW,
 static SetVector<VPIRInstruction *> collectUsersInExitBlock(
     Loop *OrigLoop, VPRecipeBuilder &Builder, VPlan &Plan,
     const MapVector<PHINode *, InductionDescriptor> &Inductions) {
-  auto *MiddleVPBB =
-      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+  auto *MiddleVPBB = Plan.getMiddleBlock();
   // No edge from the middle block to the unique exit block has been inserted
   // and there is nothing to fix from vector loop; phis should have incoming
   // from scalar loop only.
@@ -8825,8 +8823,7 @@ addUsersInExitBlock(VPlan &Plan,
   if (ExitUsersToFix.empty())
     return;
 
-  auto *MiddleVPBB =
-      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+  auto *MiddleVPBB = Plan.getMiddleBlock();
   VPBuilder B(MiddleVPBB, MiddleVPBB->getFirstNonPhi());
 
   // Introduce extract for exiting values and update the VPIRInstructions
@@ -8863,7 +8860,7 @@ static void addLiveOutsForFirstOrderRecurrences(
   // TODO: Should be replaced by
   // Plan->getScalarLoopRegion()->getSinglePredecessor() in the future once the
   // scalar region is modeled as well.
-  auto *MiddleVPBB = cast<VPBasicBlock>(VectorRegion->getSingleSuccessor());
+  auto *MiddleVPBB = Plan.getMiddleBlock();
   VPBasicBlock *ScalarPHVPBB = nullptr;
   if (MiddleVPBB->getNumSuccessors() == 2) {
     // Order is strict: first is the exit block, second is the scalar preheader.
@@ -9065,8 +9062,7 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
         bool NeedsBlends = BB != HeaderBB && !BB->phis().empty();
         return Legal->blockNeedsPredication(BB) || NeedsBlends;
       });
-  auto *MiddleVPBB =
-      cast<VPBasicBlock>(Plan->getVectorLoopRegion()->getSingleSuccessor());
+  auto *MiddleVPBB = Plan->getMiddleBlock();
   VPBasicBlock::iterator MBIP = MiddleVPBB->getFirstNonPhi();
   for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {
     // Relevant instructions from basic block BB will be grouped into VPRecipe
@@ -9277,8 +9273,7 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
   using namespace VPlanPatternMatch;
   VPRegionBlock *VectorLoopRegion = Plan->getVectorLoopRegion();
   VPBasicBlock *Header = VectorLoopRegion->getEntryBasicBlock();
-  VPBasicBlock *MiddleVPBB =
-      cast<VPBasicBlock>(VectorLoopRegion->getSingleSuccessor());
+  VPBasicBlock *MiddleVPBB = Plan->getMiddleBlock();
   for (VPRecipeBase &R : Header->phis()) {
     auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&R);
     if (!PhiR || !PhiR->isInLoop() || (MinVF.isScalar() && !PhiR->isOrdered()))

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1030,8 +1030,8 @@ void VPlan::execute(VPTransformState *State) {
   // skeleton creation, so we can only create the VPIRBasicBlocks now during
   // VPlan execution rather than earlier during VPlan construction.
   BasicBlock *MiddleBB = State->CFG.ExitBB;
-  VPBasicBlock *MiddleVPBB =
-      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  VPBasicBlock *MiddleVPBB = getMiddleBlock();
+
   // Find the VPBB for the scalar preheader, relying on the current structure
   // when creating the middle block and its successrs: if there's a single
   // predecessor, it must be the scalar preheader. Otherwise, the second

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3689,6 +3689,14 @@ class VPlan {
   VPBasicBlock *getEntry() { return Entry; }
   const VPBasicBlock *getEntry() const { return Entry; }
 
+  /// Methods to support access to the middle block.
+  const VPBasicBlock *getMiddleBlock() const {
+    return cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  }
+  VPBasicBlock *getMiddleBlock() {
+    return cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  }
+
   /// The trip count of the original loop.
   VPValue *getTripCount() const {
     assert(TripCount && "trip count needs to be set before accessing it");

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -212,8 +212,7 @@ bool VPRecipeBase::mayHaveSideEffects() const {
 
 void VPLiveOut::fixPhi(VPlan &Plan, VPTransformState &State) {
   VPValue *ExitValue = getOperand(0);
-  VPBasicBlock *MiddleVPBB =
-      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+  VPBasicBlock *MiddleVPBB = Plan.getMiddleBlock();
   VPRecipeBase *ExitingRecipe = ExitValue->getDefiningRecipe();
   auto *ExitingVPBB = ExitingRecipe ? ExitingRecipe->getParent() : nullptr;
   // Values leaving the vector loop reach live out phi's in the exiting block

llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp

@@ -248,8 +248,7 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
     return false;
   }
 
-  VPBlockBase *MiddleBB =
-      IRBB->getPlan()->getVectorLoopRegion()->getSingleSuccessor();
+  const VPBasicBlock *MiddleBB = IRBB->getPlan()->getMiddleBlock();
   if (IRBB != IRBB->getPlan()->getPreheader() &&
       IRBB->getSinglePredecessor() != MiddleBB) {
     errs() << "VPIRBasicBlock can only be used as pre-header or a successor of "