[VPlan] Create header phis once regions have been created (NFC).

Since 1b29ac1, regions are constructed
as part of the scalar transforms; moving header phi creation after
region creation slightly simplifies the code.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8045,16 +8045,17 @@ void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
       OrigLoop, *LI, Legal->getWidestInductionType(),
       getDebugLocFromInstOrOperands(Legal->getPrimaryInduction()), PSE, &LVer);
 
+  VPlanTransforms::simplifyRecipes(*VPlan0);
+  VPlanTransforms::handleEarlyExits(*VPlan0, Legal->hasUncountableEarlyExit());
+  VPlanTransforms::addMiddleCheck(*VPlan0, CM.foldTailByMasking());
+  RUN_VPLAN_PASS_NO_VERIFY(VPlanTransforms::createLoopRegions, *VPlan0);
+
   // Create recipes for header phis.
   VPlanTransforms::createHeaderPhiRecipes(
       *VPlan0, PSE, *OrigLoop, Legal->getInductionVars(),
       Legal->getReductionVars(), Legal->getFixedOrderRecurrences(),
       CM.getInLoopReductions(), Hints.allowReordering());
 
-  VPlanTransforms::simplifyRecipes(*VPlan0);
-  VPlanTransforms::handleEarlyExits(*VPlan0, Legal->hasUncountableEarlyExit());
-  VPlanTransforms::addMiddleCheck(*VPlan0, CM.foldTailByMasking());
-  RUN_VPLAN_PASS_NO_VERIFY(VPlanTransforms::createLoopRegions, *VPlan0);
   if (CM.foldTailByMasking())
     RUN_VPLAN_PASS_NO_VERIFY(VPlanTransforms::foldTailByMasking, *VPlan0);
   RUN_VPLAN_PASS_NO_VERIFY(VPlanTransforms::introduceMasksAndLinearize,
@@ -8345,17 +8346,18 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlan(VFRange &Range) {
       OrigLoop, *LI, Legal->getWidestInductionType(),
       getDebugLocFromInstOrOperands(Legal->getPrimaryInduction()), PSE);
 
-  VPlanTransforms::createHeaderPhiRecipes(
-      *Plan, PSE, *OrigLoop, Legal->getInductionVars(),
-      MapVector<PHINode *, RecurrenceDescriptor>(),
-      SmallPtrSet<const PHINode *, 1>(), SmallPtrSet<PHINode *, 1>(),
-      /*AllowReordering=*/false);
   VPlanTransforms::handleEarlyExits(*Plan,
                                     /*HasUncountableExit*/ false);
   VPlanTransforms::addMiddleCheck(*Plan, /*TailFolded*/ false);
 
   VPlanTransforms::createLoopRegions(*Plan);
 
+  VPlanTransforms::createHeaderPhiRecipes(
+      *Plan, PSE, *OrigLoop, Legal->getInductionVars(),
+      MapVector<PHINode *, RecurrenceDescriptor>(),
+      SmallPtrSet<const PHINode *, 1>(), SmallPtrSet<PHINode *, 1>(),
+      /*AllowReordering=*/false);
+
   for (ElementCount VF : Range)
     Plan->addVF(VF);
 

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -702,12 +702,7 @@ void VPlanTransforms::createHeaderPhiRecipes(
     const MapVector<PHINode *, RecurrenceDescriptor> &Reductions,
     const SmallPtrSetImpl<const PHINode *> &FixedOrderRecurrences,
     const SmallPtrSetImpl<PHINode *> &InLoopReductions, bool AllowReordering) {
-  // Retrieve the header manually from the intial plain-CFG VPlan.
-  VPBasicBlock *HeaderVPBB = cast<VPBasicBlock>(
-      Plan.getEntry()->getSuccessors()[1]->getSingleSuccessor());
-  assert(VPDominatorTree(Plan).dominates(HeaderVPBB,
-                                         HeaderVPBB->getPredecessors()[1]) &&
-         "header must dominate its latch");
+  VPBasicBlock *HeaderVPBB = Plan.getVectorLoopRegion()->getEntryBasicBlock();
 
   auto CreateHeaderPhiRecipe = [&](VPPhi *PhiR) -> VPHeaderPHIRecipe * {
     // TODO: Gradually replace uses of underlying instruction by analyses on