Restore preprocess_groupclause()

Restore preprocess_groupclause()

0452b461bc made optimizer explore alternative orderings of group-by pathkeys.
It eliminated preprocess_groupclause(), which was intended to match items
between GROUP BY and ORDER BY.  Instead, get_useful_group_keys_orderings()
function generates orderings of GROUP BY elements at the time of grouping
paths generation.  The get_useful_group_keys_orderings() function takes into
account 3 orderings of GROUP BY pathkeys and clauses: original order as written
in GROUP BY, matching ORDER BY clauses as much as possible, and matching the
input path as much as possible.  Given that even before 0452b461b,
preprocess_groupclause() could change the original order of GROUP BY clauses
we don't need to consider it apart from ordering matching ORDER BY clauses.

This commit restores preprocess_groupclause() to provide an ordering of
GROUP BY elements matching ORDER BY before generation of paths.  The new
version of preprocess_groupclause() takes into account an incremental sort.
The get_useful_group_keys_orderings() function now takes into 2 orderings of
GROUP BY elements: the order generated preprocess_groupclause() and the order
matching the input path as much as possible.

Discussion: https://postgr.es/m/CAPpHfdvyWLMGwvxaf%3D7KAp-z-4mxbSH8ti2f6mNOQv5metZFzg%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Andrei Lepikhov, Pavel Borisov