[VPlan] Track VectorPH during skeleton creation. (NFC)

Split off from llvm#108378.

This ensures that the logic works even if now vector region exits.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -479,7 +479,8 @@ class InnerLoopVectorizer {
         AC(AC), ORE(ORE), VF(VecWidth),
         MinProfitableTripCount(MinProfitableTripCount), UF(UnrollFactor),
         Builder(PSE.getSE()->getContext()), Legal(LVL), Cost(CM), BFI(BFI),
-        PSI(PSI), RTChecks(RTChecks), Plan(Plan) {
+        PSI(PSI), RTChecks(RTChecks), Plan(Plan),
+        VectorPHVPB(Plan.getEntry()->getSingleSuccessor()) {
     // Query this against the original loop and save it here because the profile
     // of the original loop header may change as the transformation happens.
     OptForSizeBasedOnProfile = llvm::shouldOptimizeForSize(
@@ -582,6 +583,11 @@ class InnerLoopVectorizer {
   virtual void printDebugTracesAtStart() {}
   virtual void printDebugTracesAtEnd() {}
 
+  /// Introduces a new VPIRBasicBlock for \p CheckIRBB to Plan between the
+  /// vector preheader and its predecessor, also connecting the new block to the
+  /// scalar preheader.
+  void introduceCheckBlockInVPlan(BasicBlock *CheckIRBB);
+
   /// The original loop.
   Loop *OrigLoop;
 
@@ -676,6 +682,10 @@ class InnerLoopVectorizer {
   BasicBlock *AdditionalBypassBlock = nullptr;
 
   VPlan &Plan;
+
+  /// The vector preheader block of \p Plan, used as target for check blocks
+  /// introduced during skeleton creation.
+  VPBlockBase *VectorPHVPB;
 };
 
 /// Encapsulate information regarding vectorization of a loop and its epilogue.
@@ -2443,19 +2453,15 @@ InnerLoopVectorizer::getOrCreateVectorTripCount(BasicBlock *InsertBlock) {
   return VectorTripCount;
 }
 
-/// Introduces a new VPIRBasicBlock for \p CheckIRBB to \p Plan between the
-/// vector preheader and its predecessor, also connecting the new block to the
-/// scalar preheader.
-static void introduceCheckBlockInVPlan(VPlan &Plan, BasicBlock *CheckIRBB) {
+void InnerLoopVectorizer::introduceCheckBlockInVPlan(BasicBlock *CheckIRBB) {
   VPBlockBase *ScalarPH = Plan.getScalarPreheader();
-  VPBlockBase *VectorPH = Plan.getVectorPreheader();
-  VPBlockBase *PreVectorPH = VectorPH->getSinglePredecessor();
+  VPBlockBase *PreVectorPH = VectorPHVPB->getSinglePredecessor();
   if (PreVectorPH->getNumSuccessors() != 1) {
     assert(PreVectorPH->getNumSuccessors() == 2 && "Expected 2 successors");
     assert(PreVectorPH->getSuccessors()[0] == ScalarPH &&
            "Unexpected successor");
     VPIRBasicBlock *CheckVPIRBB = Plan.createVPIRBasicBlock(CheckIRBB);
-    VPBlockUtils::insertOnEdge(PreVectorPH, VectorPH, CheckVPIRBB);
+    VPBlockUtils::insertOnEdge(PreVectorPH, VectorPHVPB, CheckVPIRBB);
     PreVectorPH = CheckVPIRBB;
   }
   VPBlockUtils::connectBlocks(PreVectorPH, ScalarPH);
@@ -2544,7 +2550,7 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
   LoopBypassBlocks.push_back(TCCheckBlock);
 
   // TODO: Wrap LoopVectorPreHeader in VPIRBasicBlock here.
-  introduceCheckBlockInVPlan(Plan, TCCheckBlock);
+  introduceCheckBlockInVPlan(TCCheckBlock);
 }
 
 BasicBlock *InnerLoopVectorizer::emitSCEVChecks(BasicBlock *Bypass) {
@@ -2562,7 +2568,7 @@ BasicBlock *InnerLoopVectorizer::emitSCEVChecks(BasicBlock *Bypass) {
   LoopBypassBlocks.push_back(SCEVCheckBlock);
   AddedSafetyChecks = true;
 
-  introduceCheckBlockInVPlan(Plan, SCEVCheckBlock);
+  introduceCheckBlockInVPlan(SCEVCheckBlock);
   return SCEVCheckBlock;
 }
 
@@ -2599,7 +2605,7 @@ BasicBlock *InnerLoopVectorizer::emitMemRuntimeChecks(BasicBlock *Bypass) {
 
   AddedSafetyChecks = true;
 
-  introduceCheckBlockInVPlan(Plan, MemCheckBlock);
+  introduceCheckBlockInVPlan(MemCheckBlock);
   return MemCheckBlock;
 }
 
@@ -7952,7 +7958,7 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
 
-  introduceCheckBlockInVPlan(Plan, TCCheckBlock);
+  introduceCheckBlockInVPlan(TCCheckBlock);
   return TCCheckBlock;
 }
 
@@ -8092,7 +8098,7 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
   Plan.setEntry(NewEntry);
   // OldEntry is now dead and will be cleaned up when the plan gets destroyed.
 
-  introduceCheckBlockInVPlan(Plan, Insert);
+  introduceCheckBlockInVPlan(Insert);
   return Insert;
 }