isl_multi_union_pw_aff_intersect_range: extract out 0D handling
isl_multi_union_pw_aff_intersect_range: extract out 0D handling

This will make it easier to add support for explicit domains.

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

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