isl_union_map_compute_flow: return accesses with no source

isl_union_map_compute_flow: return accesses with no source

And similarly for isl_flow_get_no_source.
Before, we would return the iterations where those accesses occur,
but if more than one element is accessed from within the same iteration,
then this does not provide enough information.

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