isl_map_affine_hull: avoid computing explicit representations for divs

isl_map_affine_hull: avoid computing explicit representations for divs

If an existentially quantified variable is not determined by an equality
then it should have little effect on the overall affine hull.
There should therefor be no need to compute explicit representations
for these existentially quantified variables, which is a possibly costly
operation and which can result in large number of basic maps.

Instead, we first compute the affine hull of each basic map separately,
extract an explicit representation for existentially quantified variables
determined by an equality and then remove all unknown divs before aligning
the divs over the basic maps.

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