isl_pw_*_union_add_aligned: use isl_pw_*_check_equal_space
isl_pw_*_union_add_aligned: use isl_pw_*_check_equal_space

This reduces the dependence on the internal representation.

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

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