[Vectorize] Avoid repeated hash lookups (NFC) (#132661)

Co-authored-by: Florian Hahn <[email protected]>

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5813,12 +5813,12 @@ LoopVectorizationCostModel::getReductionPatternCost(Instruction *I,
 
   // Test if the found instruction is a reduction, and if not return an invalid
   // cost specifying the parent to use the original cost modelling.
-  if (!InLoopReductionImmediateChains.count(RetI))
+  Instruction *LastChain = InLoopReductionImmediateChains.lookup(RetI);
+  if (!LastChain)
     return std::nullopt;
 
   // Find the reduction this chain is a part of and calculate the basic cost of
   // the reduction on its own.
-  Instruction *LastChain = InLoopReductionImmediateChains.at(RetI);
   Instruction *ReductionPhi = LastChain;
   while (!isa<PHINode>(ReductionPhi))
     ReductionPhi = InLoopReductionImmediateChains.at(ReductionPhi);