isl_map_coalesce: allow wrapping in sets that stick out in different directions

isl_map_coalesce: allow wrapping in sets that stick out in different directions

Before, we would only wrap in sets that extend beyond exactly
one facet of a given set and such that the wrapped in set itself
has a facet that is adjacent to this facet.
We now extend this process in several ways.
First, the wrapped-in set is allowed to extend beyond several facets.
Second, the wrapped-in set need not have facets adjacent to these facets.
It is sufficient that the adjacent facets are valid for the wrapped-in set.
Finally, the wrapped-in set is allowed to be of a lower dimension.