Don't convert from string to irep_idt unnecessarily

Author: NathanJPhillips

src/taint-analysis/taint_program.cpp

@@ -36,8 +36,8 @@ static void build_NONDET_retvals_replacements(
       const code_function_callt &fn_call=to_code_function_call(instr_it->code);
       if(fn_call.function().id()!=ID_symbol)
         continue;
-      const std::string callee_ident=
-        as_string(to_symbol_expr(fn_call.function()).get_identifier());
+      const irep_idt &callee_ident=
+        to_symbol_expr(fn_call.function()).get_identifier();
 
       const code_typet &fn_type=
         model.goto_functions.function_map.at(callee_ident).type;
@@ -53,7 +53,7 @@ static void build_NONDET_retvals_replacements(
           next_instr_it,
           find_or_create_symbol_expr(
             ns,
-            callee_ident+RETURN_VALUE_SUFFIX,
+            as_string(callee_ident)+RETURN_VALUE_SUFFIX,
             fn_type.return_type())
         });
     }