isl_map_coalesce: try and coalesce basic maps with different integer divisions

isl_map_coalesce: try and coalesce basic maps with different integer divisions

If two basic maps have unknown or different integer divisions,
then it may still be possible to coalesce them if the existentially
quantified variables of one basic map happen to satisfy or be adjacent to
the constraints on the quantified variables of the other basic map.
In principle we should consider all possible matchings between
existentially quantified variables or use heuristics to determine
which existentially quantified variable in one basic map should
be matched to which existentially quantified variable in the other
basic map.  Currently, however, we only consider the given order
of the existentially quantified variables, which is already
an improvement compared to not allowing unknown or different
integer divisions at all.  We do require that the total number
of existentially quantified variables is the same in both
basic maps.  Note that this results in more pairs of basic maps
being considered for coalescing, but if the default matching
between existentially quantified variables is not fruitful,
then a separating constraint should be found fairly quickly.

The previous two commits made sure that the different ways
of combining two basic maps into one can handle the case
where the integer divisions are different in the two input basic maps.

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