pgsql: Push target list evaluation through Gather Merge.

Push target list evaluation through Gather Merge.

We already do this for Gather, but it got overlooked for Gather Merge.

Amit Kapila, with review and minor revisions by Rushabh Lathia
and by me.

Discussion: http://postgr.es/m/CAA4eK1KUC5Uyu7qaifxrjpHxbSeoQh3yzwN3bThnJsmJcZ-qtA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/44ae64c388bde6e4b077272570c84dedfb17bed3

Modified Files
--------------
src/backend/optimizer/plan/planner.c          |  3 +-
src/backend/optimizer/util/pathnode.c         | 45 +++++++++++++++++----------
src/test/regress/expected/select_parallel.out | 25 +++++++++++++++
src/test/regress/sql/select_parallel.sql      | 13 ++++++++
4 files changed, 69 insertions(+), 17 deletions(-)