isl_input.c: isl_stream_read_union_set: extract out extract_union_set
isl_input.c: isl_stream_read_union_set: extract out extract_union_set

This function will be reused to read the optional explicit domain
of a zero-dimensional isl_multi_union_pw_aff, when
support for such explicit domains is added.

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

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