cloog_domain_cube: reimplement using documented functions

The original implementation used the undocumented
isl_basic_set_interval function, which will be removed
in the next release of isl.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>
Signed-off-by: Cedric Bastoul <cedric.bastoul@unistra.fr>