get_condition causes overlow for sign extended constants

[Joern Rennecke]

>         case GE:
>           if (const_val
>               != (((HOST_WIDE_INT) 1
> 
> Wouldn't changing this "1" to "-1" also fix it and in a cleaner way?
> If that works, please send me the resulting patch.

It would fix the problem I pointed out, but it would create new problems.
If op0 has a narrower mode than HOST_WIDE_INT, overflow will not be
detected by your version.