[InstCombine] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -4179,13 +4179,14 @@ Instruction *InstCombinerImpl::visitCallBase(CallBase &Call) {
     DenseMap<Value *, unsigned> Val2Idx;
     std::vector<Value *> NewLiveGc;
     for (Value *V : Bundle->Inputs) {
-      if (Val2Idx.count(V))
+      auto [It, Inserted] = Val2Idx.try_emplace(V);
+      if (!Inserted)
         continue;
       if (LiveGcValues.count(V)) {
-        Val2Idx[V] = NewLiveGc.size();
+        It->second = NewLiveGc.size();
         NewLiveGc.push_back(V);
       } else
-        Val2Idx[V] = NumOfGCLives;
+        It->second = NumOfGCLives;
     }
     // Update all gc.relocates
     for (const GCRelocateInst *Reloc : GCSP.getGCRelocates()) {