pgsql: Eliminate code duplication in replace_rte_variables callbacks

Eliminate code duplication in replace_rte_variables callbacks

The callback functions ReplaceVarsFromTargetList_callback and
pullup_replace_vars_callback are both used to replace Vars in an
expression tree that reference a particular RTE with items from a
targetlist, and they both need to expand whole-tuple references and
deal with OLD/NEW RETURNING list Vars.  As a result, currently there
is significant code duplication between these two functions.

This patch introduces a new function, ReplaceVarFromTargetList, to
perform the replacement and calls it from both callback functions,
thereby eliminating code duplication.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWhr=FM4X5kCPvVs-g2XEk+ceLsNtBK_zZMkqFn9vUjsw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/363a6e8c6fcf9f3e19fe673ae02554645974a388

Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c | 141 +++++-------------------------
src/backend/rewrite/rewriteManip.c        |  88 ++++++++++++++-----
src/include/rewrite/rewriteManip.h        |   9 +-
3 files changed, 96 insertions(+), 142 deletions(-)