isl_union_map.c: subtract_entry: use isl_map_get_space
isl_union_map.c: subtract_entry: use isl_map_get_space

This reduces the dependence on the internal representation.

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

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