pgsql: Use ExecGetCommonSlotOps infrastructure in more places.

Use ExecGetCommonSlotOps infrastructure in more places.

Append, MergeAppend, and RecursiveUnion can all use the support
functions added in commit 276279295.  The first two can report a
fixed result slot type if all their children return the same fixed
slot type.  That does nothing for the append step itself, but might
allow optimizations in the parent plan node.  RecursiveUnion can
optimize tuple hash table operations in the same way as SetOp now
does.

Patch by me; thanks to Richard Guo and David Rowley for review.

Discussion: https://postgr.es/m/1850138.1731549611@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f0b900086a370e45f730138d55da4f260d24809c

Modified Files
--------------
src/backend/executor/nodeAppend.c         | 32 ++++++++++++++++++--------
src/backend/executor/nodeMergeAppend.c    | 38 +++++++++++++++++++++----------
src/backend/executor/nodeRecursiveunion.c |  8 +++++--
3 files changed, 55 insertions(+), 23 deletions(-)