isl_tab_basic_set_non_trivial_lexmin: only add zeroing out constraints once
isl_tab_basic_set_non_trivial_lexmin: only add zeroing out constraints once

In particular, only add those constraints once at each backtracking level.
The call to isl_tab_extend_cons only allocates room for n_op
equality constraints, but if a solution is found in multiple
parts at a given level, then some of the equality constraints
could end up being added multiple times.
Only add them once to ensure there is enough room.
The alternative would be to add extra room to account
for the duplicate equality constraints.

While the multiple addition of these equality constraints did
happen in practice, it seems that the extra buffer allocated
for the slicing constraints was enough to also hold
the duplicate equality constraints.
However, the code should not rely on such lucky coincidence.

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

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