Some revises in adding sorting path

While reviewing [1], I visited other places where sorting is needed, and
have some findings.

In add_paths_with_pathkeys_for_rel, we do not try incremental sort atop
of the epq_path, which I think we can do.  I'm not sure how useful this
is in real world since the epq_path is used only for EPQ checks, but it
seems doing that doesn't cost too much.

In create_ordered_paths, we are trying to sort the cheapest partial path
and incremental sort on any partial paths with presorted keys, and then
use Gather Merge.  If the cheapest partial path is not completely sorted
but happens to have presorted keys, we would create a full sort path and
an incremental sort path on it.  I think this is not what we want.  We
are supposed to only create an incremental sort path if there are
presorted keys.

In gather_grouping_paths, we have the same issue.  In addition, for the
incremental sort paths created atop partial paths, we neglect to
calculate 'total_groups' before we use it in create_gather_merge_path.

[1]
https://www.postgresql.org/message-id/flat/CAApHDvo8Lz2H%3D42urBbfP65LTcEUOh288MT7DsG2_EWtW1AXHQ%40mail.gmail.com

Thanks
Richard