isl_tab_basic_set_non_trivial_lexmin: skip known impossible cases
isl_tab_basic_set_non_trivial_lexmin: skip known impossible cases

While stepping through all the cases that ensure
non-triviality of the solution, equality constraints
are added after each pair of cases and they need to be satisfied
by all subsequent cases.
If these equality constraints render the problem infeasible,
then there is no point in looking at any further cases at this level.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,PollyLabs/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,inducer/isl-mirror,nicolasvasilache/isl,PollyLabs/isl