pgsql: Fix planner failure in some cases of sorting by an aggregate.

Fix planner failure in some cases of sorting by an aggregate.

An oversight introduced by the incremental-sort patches caused
"could not find pathkey item to sort" errors in some situations
where a sort key involves an aggregate or window function.

The basic problem here is that find_em_expr_usable_for_sorting_rel
isn't properly modeling what prepare_sort_from_pathkeys will do
later. Rather than hoping we can keep those functions in sync,
let's refactor so that they actually share the code for
identifying a suitable sort expression.

With this refactoring, tlist.c's tlist_member_ignore_relabel
is unused. I removed it in HEAD but left it in place in v13,
in case any extensions are using it.

Per report from Luc Vlaming. Back-patch to v13 where the
problem arose.

James Coleman and Tom Lane

Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7bfba4f1933003716d432d29d4d228bcf28e2e70

Modified Files
--------------
src/backend/optimizer/path/equivclass.c | 255 ++++++++++++++++++++-----
src/backend/optimizer/plan/createplan.c | 118 +-----------
src/include/optimizer/paths.h | 8 +
src/test/regress/expected/incremental_sort.out | 26 +++
src/test/regress/sql/incremental_sort.sql | 7 +
5 files changed, 262 insertions(+), 152 deletions(-)