[VPlan] Check successors in VPlan to check if scalar epi required (NFC)

Now that the branches to the scalar epilogue are modeled in VPlan
directly, check the VPlan to see if a scalar epilogue is required.

Preparation for #100658.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8545,13 +8545,18 @@ static void addCanonicalIVRecipes(VPlan &Plan, Type *IdxTy, bool HasNUW,
 static void addUsersInExitBlock(
     Loop *OrigLoop, VPRecipeBuilder &Builder, VPlan &Plan,
     const MapVector<PHINode *, InductionDescriptor> &Inductions) {
-  BasicBlock *ExitBB = OrigLoop->getUniqueExitBlock();
-  BasicBlock *ExitingBB = OrigLoop->getExitingBlock();
-  // Only handle single-exit loops with unique exit blocks for now.
-  if (!ExitBB || !ExitBB->getSinglePredecessor() || !ExitingBB)
+  auto MiddleVPBB =
+      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+  // 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.
+  if (MiddleVPBB->getNumSuccessors() != 2)
     return;
 
   // Introduce VPUsers modeling the exit values.
+  BasicBlock *ExitBB =
+      cast<VPIRBasicBlock>(MiddleVPBB->getSuccessors()[0])->getIRBasicBlock();
+  BasicBlock *ExitingBB = OrigLoop->getExitingBlock();
   for (PHINode &ExitPhi : ExitBB->phis()) {
     Value *IncomingValue =
         ExitPhi.getIncomingValueForBlock(ExitingBB);
@@ -8777,13 +8782,8 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   // After here, VPBB should not be used.
   VPBB = nullptr;
 
-  if (CM.requiresScalarEpilogue(Range)) {
-    // 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.
-  } else
-    addUsersInExitBlock(OrigLoop, RecipeBuilder, *Plan,
-                        Legal->getInductionVars());
+  addUsersInExitBlock(OrigLoop, RecipeBuilder, *Plan,
+                      Legal->getInductionVars());
 
   assert(isa<VPRegionBlock>(Plan->getVectorLoopRegion()) &&
          !Plan->getVectorLoopRegion()->getEntryBasicBlock()->empty() &&

llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll

@@ -52,8 +52,6 @@ define void @vector_reverse_i64(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Scalarizing: %arrayidx3 = getelementptr inbounds i32, ptr %A, i64 %idxprom
 ; CHECK-NEXT:  LV: Scalarizing: %cmp = icmp ugt i64 %indvars.iv, 1
 ; CHECK-NEXT:  LV: Scalarizing: %indvars.iv.next = add nsw i64 %indvars.iv, -1
-; CHECK-NEXT:  LV: Loop does not require scalar epilogue
-; CHECK-NEXT:  LV: Loop does not require scalar epilogue
 ; CHECK-NEXT:  VPlan 'Initial VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF
 ; CHECK-NEXT:  Live-in vp<%1> = vector-trip-count
@@ -257,8 +255,6 @@ define void @vector_reverse_f32(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Scalarizing: %arrayidx3 = getelementptr inbounds float, ptr %A, i64 %idxprom
 ; CHECK-NEXT:  LV: Scalarizing: %cmp = icmp ugt i64 %indvars.iv, 1
 ; CHECK-NEXT:  LV: Scalarizing: %indvars.iv.next = add nsw i64 %indvars.iv, -1
-; CHECK-NEXT:  LV: Loop does not require scalar epilogue
-; CHECK-NEXT:  LV: Loop does not require scalar epilogue
 ; CHECK-NEXT:  VPlan 'Initial VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF
 ; CHECK-NEXT:  Live-in vp<%1> = vector-trip-count