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

This work is in preparation for PRs #112138 and #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 #112138 lands we'll need to do more work to discover
the middle block.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7703,8 +7703,7 @@ 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();
   if (VectorizingEpilogue)
     for (VPRecipeBase &R : *ExitVPBB) {
       fixReductionScalarResumeWhenVectorizingEpilog(
@@ -8830,8 +8829,7 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan) {
 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.
@@ -8876,8 +8874,7 @@ 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
@@ -8905,7 +8902,7 @@ static void addExitUsersForFirstOrderRecurrences(
     VPlan &Plan, SetVector<VPIRInstruction *> &ExitUsersToFix) {
   VPRegionBlock *VectorRegion = Plan.getVectorLoopRegion();
   auto *ScalarPHVPBB = Plan.getScalarPreheader();
-  auto *MiddleVPBB = cast<VPBasicBlock>(VectorRegion->getSingleSuccessor());
+  auto *MiddleVPBB = Plan.getMiddleBlock();
   VPBuilder ScalarPHBuilder(ScalarPHVPBB);
   VPBuilder MiddleBuilder(MiddleVPBB, MiddleVPBB->getFirstNonPhi());
   VPValue *TwoVPV = Plan.getOrAddLiveIn(
@@ -9085,8 +9082,7 @@ 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
@@ -9303,8 +9299,7 @@ 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

@@ -1034,8 +1034,7 @@ 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();
   BasicBlock *ScalarPh = MiddleBB->getSingleSuccessor();
   replaceVPBBWithIRVPBB(getScalarPreheader(), ScalarPh);
   replaceVPBBWithIRVPBB(MiddleVPBB, MiddleBB);

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3748,6 +3748,16 @@ class VPlan {
     return cast<VPBasicBlock>(ScalarHeader->getSinglePredecessor());
   }
 
+  /// Returns the 'middle' block of the plan, that is the block that selects
+  /// whether to execute the scalar tail loop or the exit block from the loop
+  /// latch.
+  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");