LibJS/JIT: Compile the ConcatString bytecode instruction

Author: https://github.com/skyrising
Commit: 1d3062de9e
Pull-request: https://github.com/SerenityOS/serenity/pull/21665

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -362,6 +362,8 @@ public:
     ThrowCompletionOr<void> execute_impl(Bytecode::Interpreter&) const;
     DeprecatedString to_deprecated_string_impl(Bytecode::Executable const&) const;
 
+    Register lhs() const { return m_lhs; }
+
 private:
     Register m_lhs;
 };