isl_pw_multi_aff_drop_dims: fix dropping of output dimensions
Most isl_pw_*_drop_dims functions don't support dropping of output dimensions
because their pieces have a fixed single output dimension.
However, the pieces of isl_pw_multi_* objects do support dropping of output
dimensions and therefore so should the isl_pw_multi_*_drop_dims functions.
We simply need to skip dropping dimensions from the domains.
The original code would leave the result in an inconsistent state.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>