rename isl_union_*_group_has_domain_space to ..._has_same_domain_space
rename isl_union_*_group_has_domain_space to ..._has_same_domain_space

The original name, but especially the comment,
was confusing because the domain space of the group
is not compared to the "space" argument, but
to the domain of this argument.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl