isl_pw_*_drop_dims: use isl_pw_*_{take,restore}_space
isl_pw_*_drop_dims: use isl_pw_*_{take,restore}_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