pgsql: Split QTW_EXAMINE_RTES flag into QTW_EXAMINE_RTES_BEFORE/_AFTER.

Split QTW_EXAMINE_RTES flag into QTW_EXAMINE_RTES_BEFORE/_AFTER.

This change allows callers of query_tree_walker() to choose whether
to visit an RTE before or after visiting the contents of the RTE
(i.e., prefix or postfix tree order). All existing users of
QTW_EXAMINE_RTES want the QTW_EXAMINE_RTES_BEFORE behavior, but
an upcoming patch will want QTW_EXAMINE_RTES_AFTER, and it seems
like a potentially useful change on its own.

Andreas Karlsson (extracted from CTE inlining patch)

Discussion: https://postgr.es/m/8810.1542402910@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/18c0da88a5d9da566c3bfac444366b73bd0b57da

Modified Files
--------------
src/backend/nodes/nodeFuncs.c | 14 +++++++++++---
src/backend/optimizer/plan/setrefs.c | 4 ++--
src/backend/rewrite/rewriteManip.c | 6 +++---
src/include/nodes/nodeFuncs.h | 7 +++++--
4 files changed, 21 insertions(+), 10 deletions(-)