isl_set_overlaps: implement in terms of isl_set_is_disjoint
isl_set_overlaps: implement in terms of isl_set_is_disjoint

This removes some code duplication and should be more efficient
because isl_set_is_disjoint avoids computing the entire intersection
in some cases.

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

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