isl_basic_set_lineality_space: allow local variables in input
isl_basic_set_lineality_space: allow local variables in input

Even though lineality spaces are originally defined on rational sets,
in particular sets without local variables, the same mechanism
still makes sense on sets with local variables.
Allow them to ensure that isl_basic_set_lineality_space can
be applied to any basic set.
This will allow isl_basic_set_lineality_space to be used
as a basis for an isl_union_set_combined_lineality_space,
without having to worry about local variables,
even though the initial use case for isl_union_set_combined_lineality_space
will not actually use it on sets with local variables.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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