pgsql: Make simpler-simple-expressions code cope with a Gather plan.

Make simpler-simple-expressions code cope with a Gather plan.

Commit 00418c612 expected that the plan generated for a simple-expression
query would always be a plain Result node. However, if force_parallel_mode
is on, the planner might stick a Gather atop that. Cope by looking through
the Gather. For safety, assert that the Gather's tlist is trivial.

Per buildfarm.

Discussion: /messages/by-id/23425.1502822098@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b73f1b5c29e0ace5a281bc13ce09dea30e1b66de

Modified Files
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src/pl/plpgsql/src/pl_exec.c | 61 +++++++++++++++++++++++++++++++++-----------
1 file changed, 46 insertions(+), 15 deletions(-)

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