isl_basic_map_plain_cmp: use isl_space_cmp to compare spaces
isl_basic_map_plain_cmp: use isl_space_cmp to compare spaces

This is a matter of separation of concerns.
Note that isl_space_cmp compares spaces in a more detailed way,
so this change may result in a different ordering of basic maps.

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

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