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

[Kazu Hirata via llvm-commits]

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@@ -5815,12 +5815,13 @@ 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))
+  auto RetIIt = InLoopReductionImmediateChains.find(RetI);
+  if (RetIIt == InLoopReductionImmediateChains.end())
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kazutakahirata wrote:

@fhahn Thanks!  That's a lot simpler because we never insert `nullptr` into the value side of the map.

https://github.com/llvm/llvm-project/pull/132661