[VPlan] Retrieve vector TC for epilogue from resume phi (NFC).

Instead of relying on getOrCreateVectorTripCount to initialize
EPI.VectorTripCount, delay initialization after we retrieved the resume
phi and get the trip count from there. This makes the code independent
of legacy vector trip count creation.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7530,9 +7530,6 @@ BasicBlock *EpilogueVectorizerMainLoop::createEpilogueVectorizedLoopSkeleton() {
   EPI.MainLoopIterationCountCheck =
       emitIterationCountCheck(LoopScalarPreHeader, false);
 
-  // Generate the induction variable.
-  EPI.VectorTripCount = getOrCreateVectorTripCount(LoopVectorPreHeader);
-
   replaceVPBBWithIRVPBB(Plan.getScalarPreheader(), LoopScalarPreHeader);
   return LoopVectorPreHeader;
 }
@@ -9808,7 +9805,7 @@ static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
 static void
 preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
                                  const SCEV2ValueTy &ExpandedSCEVs,
-                                 const EpilogueLoopVectorizationInfo &EPI) {
+                                 EpilogueLoopVectorizationInfo &EPI) {
   VPRegionBlock *VectorLoop = Plan.getVectorLoopRegion();
   VPBasicBlock *Header = VectorLoop->getEntryBasicBlock();
   Header->setName("vec.epilog.vector.body");
@@ -9826,30 +9823,13 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
       // loop.
       using namespace llvm::PatternMatch;
       PHINode *EPResumeVal = &*L->getLoopPreheader()->phis().begin();
-      assert(EPResumeVal->getType() == IV->getScalarType() &&
-             match(EPResumeVal->getIncomingValueForBlock(
-                       EPI.MainLoopIterationCountCheck),
-                   m_SpecificInt(0)) &&
-             EPResumeVal ==
-                 find_singleton<PHINode>(
-                     L->getLoopPreheader()->phis(),
-                     [&EPI, IV](PHINode &P, bool) -> PHINode * {
-                       if (P.getType() == IV->getScalarType() &&
-                           match(P.getIncomingValueForBlock(
-                                     EPI.MainLoopIterationCountCheck),
-                                 m_SpecificInt(0)) &&
-                           any_of(P.incoming_values(),
-                                  [&EPI](Value *Inc) {
-                                    return Inc == EPI.VectorTripCount;
-                                  }) &&
-                           all_of(P.incoming_values(), [&EPI](Value *Inc) {
-                             return Inc == EPI.VectorTripCount ||
-                                    match(Inc, m_SpecificInt(0));
-                           }))
-                         return &P;
-                       return nullptr;
-                     }) &&
-             "Epilogue resume phis do not match!");
+      for (Value *Inc : EPResumeVal->incoming_values()) {
+        if (match(Inc, m_SpecificInt(0)))
+          continue;
+        assert(!EPI.VectorTripCount &&
+               "Must only have a single non-zero incoming value");
+        EPI.VectorTripCount = Inc;
+      }
       VPValue *VPV = Plan.getOrAddLiveIn(EPResumeVal);
       assert(all_of(IV->users(),
                     [](const VPUser *U) {