isl_pw_*_set_dim_id: use isl_pw_*_get_space
isl_pw_*_set_dim_id: use isl_pw_*_get_space

This reduces the dependence on the internal representation.

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

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