[IR] Avoid repeated hash lookups (NFC) (#112469)

Author: kazutakahirata

llvm/lib/IR/LegacyPassManager.cpp

@@ -104,15 +104,13 @@ void PMDataManager::emitInstrCountChangedRemark(
       [&FunctionToInstrCount](Function &MaybeChangedFn) {
         // Update the total module count.
         unsigned FnSize = MaybeChangedFn.getInstructionCount();
-        auto It = FunctionToInstrCount.find(MaybeChangedFn.getName());
 
         // If we created a new function, then we need to add it to the map and
         // say that it changed from 0 instructions to FnSize.
-        if (It == FunctionToInstrCount.end()) {
-          FunctionToInstrCount[MaybeChangedFn.getName()] =
-              std::pair<unsigned, unsigned>(0, FnSize);
+        auto [It, Inserted] = FunctionToInstrCount.try_emplace(
+            MaybeChangedFn.getName(), 0, FnSize);
+        if (Inserted)
           return;
-        }
         // Insert the new function size into the second member of the pair. This
         // tells us whether or not this function changed in size.
         It->second.second = FnSize;