isl_basic_set_preimage: use isl_basic_set_add_dims

isl_basic_set_preimage: use isl_basic_set_add_dims

isl_basic_set_extend always returns an object living in a map space.
This happens to work out because isl_basic_set_preimage only
gets called on objects that will get there spaces replaced afterwards.
However, the objects may also interact with other objects
before this space replacement and this only works because
isl_space_underlying also returns a map space.
This behavior will be changed in the next commit,
so isl_basic_set_preimage needs to be adjusted first.
Call isl_basic_set_add_dims, which does not change sets into maps.
Note that this does mean that in the current situation,
isl_basic_set_add_dims may get called on objects living in a map space,
but this should be fairly harmless and will be fixed in the next commit.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>