Use extended interpretation of "is constant"

Author: tautschnig

src/analyses/constant_propagator.cpp

@@ -38,7 +38,7 @@ void constant_propagator_domaint::assign_rec(
   values.replace_const(tmp);
   simplify(tmp, ns);
 
-  if(tmp.is_constant())
+  if(values.is_constant(tmp))
     values.set_to(s, tmp);
   else
     values.set_to_top(s);