get_condition causes overlow for sign extended constants

[Richard Kenner]

    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.

How is that possible?  The shift amount isn't being changed.  So if the
shift of -1 would overflow, why wouldn't the current shift of 1 in the
similar situation?