add isl_basic_set_lexmin_compute_divs
One of the side effects of the parametric integer programming
algorithm is that any existentially quantified variable is
given an explicit representation corresponding to the minimal
value. isl_basic_map_compute_divs exploits this property
by calling isl_basic_set_lexmin internally.
Introduce an isl_basic_set_lexmin_compute_divs that makes
this expectation of the result being free of existentially
quantified variables (without explicit representation) more explicit.
isl_basic_set_lexmin_compute_divs then sets a flag to keep
track of this requirement. This flag, or rather the absence
of this flag being set, will be exploited in the next commit.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>