[VPlan] Introduce and use getVectorPreheader (NFC).

Introduce a dedicated function to retrieve the vector preheader. This
ensures the correct block is used, even if the skeleton is exetended.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -275,8 +275,8 @@ Value *VPTransformState::get(VPValue *Def, bool NeedsScalar) {
     // Place the code for broadcasting invariant variables in the new preheader.
     IRBuilder<>::InsertPointGuard Guard(Builder);
     if (SafeToHoist) {
-      BasicBlock *LoopVectorPreHeader = CFG.VPBB2IRBB[cast<VPBasicBlock>(
-          Plan->getVectorLoopRegion()->getSinglePredecessor())];
+      BasicBlock *LoopVectorPreHeader =
+          CFG.VPBB2IRBB[Plan->getVectorPreheader()];
       if (LoopVectorPreHeader)
         Builder.SetInsertPoint(LoopVectorPreHeader->getTerminator());
     }

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3812,6 +3812,11 @@ class VPlan {
     return cast<VPRegionBlock>(getEntry()->getSingleSuccessor());
   }
 
+  /// Returns the preheader of the vector loop region.
+  VPBasicBlock *getVectorPreheader() {
+    return cast<VPBasicBlock>(getVectorLoopRegion()->getSinglePredecessor());
+  }
+
   /// Returns the canonical induction recipe of the vector loop.
   VPCanonicalIVPHIRecipe *getCanonicalIV() {
     VPBasicBlock *EntryVPBB = getVectorLoopRegion()->getEntryBasicBlock();

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1073,9 +1073,7 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
 
 /// Move loop-invariant recipes out of the vector loop region in \p Plan.
 static void licm(VPlan &Plan) {
-  VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
-  VPBasicBlock *Preheader =
-      cast<VPBasicBlock>(LoopRegion->getSinglePredecessor());
+  VPBasicBlock *Preheader = Plan.getVectorPreheader();
 
   // Return true if we do not know how to (mechanically) hoist a given recipe
   // out of a loop region. Does not address legality concerns such as aliasing
@@ -1089,6 +1087,7 @@ static void licm(VPlan &Plan) {
   // Hoist any loop invariant recipes from the vector loop region to the
   // preheader. Preform a shallow traversal of the vector loop region, to
   // exclude recipes in replicate regions.
+  VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_shallow(LoopRegion->getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
@@ -1133,7 +1132,7 @@ void VPlanTransforms::truncateToMinimalBitwidths(
   DenseMap<VPValue *, VPWidenCastRecipe *> ProcessedTruncs;
   Type *CanonicalIVType = Plan.getCanonicalIV()->getScalarType();
   VPTypeAnalysis TypeInfo(CanonicalIVType);
-  VPBasicBlock *PH = Plan.getEntry();
+  VPBasicBlock *PH = Plan.getVectorPreheader();
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_deep(Plan.getVectorLoopRegion()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
@@ -1317,7 +1316,7 @@ static VPActiveLaneMaskPHIRecipe *addVPLaneMaskPhiAndUpdateExitBranch(
   // We can't use StartV directly in the ActiveLaneMask VPInstruction, since
   // we have to take unrolling into account. Each part needs to start at
   //   Part * VF
-  auto *VecPreheader = cast<VPBasicBlock>(TopRegion->getSinglePredecessor());
+  auto *VecPreheader = Plan.getVectorPreheader();
   VPBuilder Builder(VecPreheader);
 
   // Create the ActiveLaneMask instruction using the correct start values.