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: https://postgr.es/m/23425.1502822098@sss.pgh.pa.us