[LV] Check if plan has an early exit via plan's exit blocks. (NFC) (#134720)

Add a dedicated function to check if a plan is for a loop with an early
exit. This can easily be determined by checking the exit blocks.

This allows removing a use of Legal->hasUncountableEarlyExit() from
InnerLoopVectorizer.

PR: #134720

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7566,14 +7566,10 @@ VectorizationFactor LoopVectorizationPlanner::computeBestVF() {
   VPCostContext CostCtx(CM.TTI, *CM.TLI, Legal->getWidestInductionType(), CM,
                         CM.CostKind);
   precomputeCosts(BestPlan, BestFactor.Width, CostCtx);
-  // Set PlanForEarlyExitLoop to true if the BestPlan has been built from a
-  // loop with an uncountable early exit. The legacy cost model doesn't
-  // properly model costs for such loops.
-  bool PlanForEarlyExitLoop =
-      BestPlan.getVectorLoopRegion() &&
-      BestPlan.getVectorLoopRegion()->getSingleSuccessor() !=
-          BestPlan.getMiddleBlock();
-  assert((BestFactor.Width == LegacyVF.Width || PlanForEarlyExitLoop ||
+  // Verify that the VPlan-based and legacy cost models agree, except for VPlans
+  // with early exits and plans with additional VPlan simplifications. The
+  // legacy cost model doesn't properly model costs for such loops.
+  assert((BestFactor.Width == LegacyVF.Width || BestPlan.hasEarlyExit() ||
           planContainsAdditionalSimplifications(getPlanFor(BestFactor.Width),
                                                 CostCtx, OrigLoop) ||
           planContainsAdditionalSimplifications(getPlanFor(LegacyVF.Width),
@@ -7784,7 +7780,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   // 2.5 When vectorizing the epilogue, fix reduction resume values from the
   // additional bypass block.
   if (VectorizingEpilogue) {
-    assert(!ILV.Legal->hasUncountableEarlyExit() &&
+    assert(!BestVPlan.hasEarlyExit() &&
            "Epilogue vectorisation not yet supported with early exits");
     BasicBlock *PH = OrigLoop->getLoopPreheader();
     BasicBlock *BypassBlock = ILV.getAdditionalBypassBlock();

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3768,6 +3768,14 @@ class VPlan {
   /// successors of the block in VPlan. The returned block is owned by the VPlan
   /// and deleted once the VPlan is destroyed.
   VPIRBasicBlock *createVPIRBasicBlock(BasicBlock *IRBB);
+
+  /// Returns true if the VPlan is based on a loop with an early exit. That is
+  /// the case if the VPlan has either more than one exit block or a single exit
+  /// block with multiple predecessors (one for the exit via the latch and one
+  /// via the other early exit).
+  bool hasEarlyExit() const {
+    return ExitBlocks.size() > 1 || ExitBlocks[0]->getNumPredecessors() > 1;
+  }
 };
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)