SMV: tighten expression type checking

This tightens the rules for type checking SMV expressions.

Author: kroening

src/smvlang/smv_typecheck.cpp

@@ -1319,9 +1319,6 @@ void smv_typecheckt::convert_expr_to(exprt &expr, const typet &type)
         expr.type() = type;
         return;
       }
-
-      expr = typecast_exprt{expr, type};
-      return;
     }
     else if(type.id() == ID_bool)
     {