pet_skip_info_add_scops: add scops for computing skip conditions in sequence
They were added in parallel before in order to avoid any skip condition
on the original scop from applying to the scops for computing skip conditions.
Currently, the application of skip conditions is the only difference
between pet_scop_add_par and pet_scop_add_seq since the actual execution
order is determined by setting prefixes.
However, when we switch to schedule trees, we will no longer be keeping
track of such prefixes and pet_scop_add_par and pet_scop_add_seq
will imply different schedules.
We therefore need to call the appropriate function here to prepare
for this change.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>