[wasm] Avoid repeated hash lookups (NFC) (#122626)

Author: kazutakahirata

lld/wasm/SymbolTable.cpp

@@ -1009,8 +1009,8 @@ void SymbolTable::handleWeakUndefines() {
 }
 
 DefinedFunction *SymbolTable::createUndefinedStub(const WasmSignature &sig) {
-  if (stubFunctions.count(sig))
-    return stubFunctions[sig];
+  if (auto it = stubFunctions.find(sig); it != stubFunctions.end())
+    return it->second;
   LLVM_DEBUG(dbgs() << "createUndefinedStub: " << toString(sig) << "\n");
   auto *sym = reinterpret_cast<DefinedFunction *>(make<SymbolUnion>());
   sym->isUsedInRegularObj = true;