[TableGen] Avoid repeated hash lookups (NFC) (#120873)

Author: kazutakahirata

llvm/utils/TableGen/CallingConvEmitter.cpp

@@ -378,10 +378,11 @@ void CallingConvEmitter::emitArgRegisterLists(raw_ostream &O) {
         const std::string &InnerCCName = InnerEntry.first;
         std::set<std::string> &InnerRegisters = InnerEntry.second;
 
-        if (InnerRegisters.find(CCName) != InnerRegisters.end()) {
-          AssignedRegsMap[InnerCCName].insert(AssignedRegsMap[CCName].begin(),
-                                              AssignedRegsMap[CCName].end());
-          InnerRegisters.erase(CCName);
+        auto It = InnerRegisters.find(CCName);
+        if (It != InnerRegisters.end()) {
+          const auto &Src = AssignedRegsMap[CCName];
+          AssignedRegsMap[InnerCCName].insert(Src.begin(), Src.end());
+          InnerRegisters.erase(It);
         }
       }