[LV] Simplify finding EPResumeValue (NFC).

It should be sufficient to check that the resume phi has the correct
type, as the vector trip count as incoming value and starts at 0
otherwise. There is no need to find the middle block.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9741,25 +9741,16 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
       // VPlan.
       // FIXME: Improve modeling for canonical IV start values in the epilogue
       // loop.
-      BasicBlock *MainMiddle = find_singleton<BasicBlock>(
-          predecessors(L->getLoopPreheader()),
-          [&EPI](BasicBlock *BB, bool) -> BasicBlock * {
-            if (BB != EPI.MainLoopIterationCountCheck &&
-                BB != EPI.EpilogueIterationCountCheck &&
-                BB != EPI.SCEVSafetyCheck && BB != EPI.MemSafetyCheck)
-              return BB;
-            return nullptr;
-          });
       using namespace llvm::PatternMatch;
       Type *IdxTy = IV->getScalarType();
       PHINode *EPResumeVal = find_singleton<PHINode>(
           L->getLoopPreheader()->phis(),
-          [&EPI, IdxTy, MainMiddle](PHINode &P, bool) -> PHINode * {
+          [&EPI, IdxTy](PHINode &P, bool) -> PHINode * {
             if (P.getType() == IdxTy &&
-                P.getIncomingValueForBlock(MainMiddle) == EPI.VectorTripCount &&
                 match(
                     P.getIncomingValueForBlock(EPI.MainLoopIterationCountCheck),
-                    m_SpecificInt(0)))
+                    m_SpecificInt(0)) &&
+                is_contained(P.incoming_values(), EPI.VectorTripCount))
               return &P;
             return nullptr;
           });