Make sure code_typet is fully constructed

The existence of the parameters and return_type properties may
depend on whether parameters() or return_type() has been called
or not, which can lead to unexpectedly unequal instructions
in goto programs.

src/util/std_types.h

@@ -760,6 +760,10 @@ class code_typet:public typet
 public:
   code_typet():typet(ID_code)
   {
+    // make sure these properties are always there to avoid problems
+    // with irept comparisons
+    add(ID_parameters);
+    add_type(ID_return_type);
   }
 
   // used to be argumentt -- now uses standard terminology