[VPlan] Split up attachCheckBlock in distinct helpers for re-use (NFC).

Split up attachCheckBlock into its distinct operations:
* inserting the check block in the CFG + updating phis, and
* adding the branch VPInstruction.

Those helpers can be re-used in follow-up changes.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -966,38 +966,37 @@ void VPlanTransforms::createLoopRegions(VPlan &Plan) {
   TopRegion->getEntryBasicBlock()->setName("vector.body");
 }
 
-// Likelyhood of bypassing the vectorized loop due to a runtime check block,
-// including memory overlap checks block and wrapping/unit-stride checks block.
-static constexpr uint32_t CheckBypassWeights[] = {1, 127};
-
-void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
-                                       BasicBlock *CheckBlock,
-                                       bool AddBranchWeights) {
-  VPValue *CondVPV = Plan.getOrAddLiveIn(Cond);
-  VPBasicBlock *CheckBlockVPBB = Plan.createVPIRBasicBlock(CheckBlock);
+/// Insert \p CheckBlockVPBB on the edge leading to the vector preheader,
+/// connecting it to both vector and scalar preheaders. Updates scalar
+/// preheader phis to account for the new predecessor.
+static void insertCheckBlockBeforeVectorLoop(VPlan &Plan,
+                                             VPBasicBlock *CheckBlockVPBB) {
   VPBlockBase *VectorPH = Plan.getVectorPreheader();
-  VPBlockBase *ScalarPH = Plan.getScalarPreheader();
+  auto *ScalarPH = cast<VPBasicBlock>(Plan.getScalarPreheader());
   VPBlockBase *PreVectorPH = VectorPH->getSinglePredecessor();
   VPBlockUtils::insertOnEdge(PreVectorPH, VectorPH, CheckBlockVPBB);
   VPBlockUtils::connectBlocks(CheckBlockVPBB, ScalarPH);
   CheckBlockVPBB->swapSuccessors();
-
-  // We just connected a new block to the scalar preheader. Update all
-  // VPPhis by adding an incoming value for it, replicating the last value.
-  unsigned NumPredecessors = ScalarPH->getNumPredecessors();
-  for (VPRecipeBase &R : cast<VPBasicBlock>(ScalarPH)->phis()) {
-    assert(isa<VPPhi>(&R) && "Phi expected to be VPPhi");
-    assert(cast<VPPhi>(&R)->getNumIncoming() == NumPredecessors - 1 &&
-           "must have incoming values for all operands");
-    R.addOperand(R.getOperand(NumPredecessors - 2));
+  unsigned NumPreds = ScalarPH->getNumPredecessors();
+  for (VPRecipeBase &R : ScalarPH->phis()) {
+    auto *Phi = cast<VPPhi>(&R);
+    assert(Phi->getNumIncoming() == NumPreds - 1 &&
+           "must have incoming values for all predecessors");
+    Phi->addOperand(Phi->getOperand(NumPreds - 2));
   }
+}
 
-  VPIRMetadata VPBranchWeights;
-  auto *Term =
-      VPBuilder(CheckBlockVPBB)
-          .createNaryOp(
-              VPInstruction::BranchOnCond, {CondVPV},
-              Plan.getVectorLoopRegion()->getCanonicalIV()->getDebugLoc());
+// Likelyhood of bypassing the vectorized loop due to a runtime check block,
+// including memory overlap checks block and wrapping/unit-stride checks block.
+static constexpr uint32_t CheckBypassWeights[] = {1, 127};
+
+/// Create a BranchOnCond terminator in \p CheckBlockVPBB. Optionally adds
+/// branch weights.
+static void addBypassBranch(VPlan &Plan, VPBasicBlock *CheckBlockVPBB,
+                            VPValue *Cond, bool AddBranchWeights) {
+  DebugLoc DL = Plan.getVectorLoopRegion()->getCanonicalIV()->getDebugLoc();
+  auto *Term = VPBuilder(CheckBlockVPBB)
+                   .createNaryOp(VPInstruction::BranchOnCond, {Cond}, DL);
   if (AddBranchWeights) {
     MDBuilder MDB(Plan.getContext());
     MDNode *BranchWeights =
@@ -1006,6 +1005,15 @@ void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
   }
 }
 
+void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
+                                       BasicBlock *CheckBlock,
+                                       bool AddBranchWeights) {
+  VPValue *CondVPV = Plan.getOrAddLiveIn(Cond);
+  VPBasicBlock *CheckBlockVPBB = Plan.createVPIRBasicBlock(CheckBlock);
+  insertCheckBlockBeforeVectorLoop(Plan, CheckBlockVPBB);
+  addBypassBranch(Plan, CheckBlockVPBB, CondVPV, AddBranchWeights);
+}
+
 void VPlanTransforms::addMinimumIterationCheck(
     VPlan &Plan, ElementCount VF, unsigned UF,
     ElementCount MinProfitableTripCount, bool RequiresScalarEpilogue,