isl_ast_build_ast_from_schedule: properly handle union atomic options
In particular, if the domain for which a given dimension should
be atomic is a union of basic sets, then the actual atomic domain
may be larger than this union and we need to make sure that this
larger set is removed from the remainder domain to preserve
the invariant that the elements of the list computed by compute_domains
are disjoint.
Reported-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>