isl_basic_set_sample: perform basis reduction at each level by default

isl_basic_set_sample: perform basis reduction at each level by default

Although always performing basis reduction can lead to slow-downs
on larger problems, only performing it once can result in some
relatively simple problems taking an inordinate amount of time.
We therefore revert back to the default from before 8db0a80
(isl_basic_set_sample: only perform basis reduction once,
Sun Aug 9 11:42:41 2009 +0200).

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>