isl_pw_*_un_op: use isl_pw_*_{take,restore}_base_at
isl_pw_*_un_op: use isl_pw_*_{take,restore}_base_at

This reduces the dependence on the internal representation.

Note that the call to isl_pw_*_cow can be removed
because it is called by isl_pw_*_restore_base_at when needed.

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

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