pgsql: Simplify the planner's new representation of indexable clauses a

Simplify the planner's new representation of indexable clauses a little.

In commit 1a8d5afb0, I thought it'd be a good idea to define
IndexClause.indexquals as NIL in the most common case where the given
clause (IndexClause.rinfo) is usable exactly as-is. It'd be more
consistent to define the indexquals in that case as being a one-element
list containing IndexClause.rinfo, but I thought saving the palloc
overhead for making such a list would be worthwhile.

In hindsight, that was a great example of "premature optimization is the
root of all evil": it's complicated everyplace that needs to deal with
the indexquals, requiring duplicative code to handle both the simple
case and the not-simple case. I'd initially found that tolerable but
it's getting less so as I mop up some areas that I'd not touched in
1a8d5afb0. In any case, two more pallocs during a planner run are
surely at the noise level (a conclusion confirmed by a bit of
microbenchmarking). So let's change this decision before it becomes
set in stone, and insist that IndexClause.indexquals always be a valid
list of the actual index quals for the clause.

Discussion: https://postgr.es/m/24586.1550106354@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8fd3fdd85a3e22f04b2cb0949450f63cb48952cd

Modified Files
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src/backend/optimizer/path/indxpath.c | 8 +--
src/backend/optimizer/plan/createplan.c | 30 ++-------
src/backend/utils/adt/selfuncs.c | 112 ++++++++++++--------------------
src/include/nodes/pathnodes.h | 26 +++-----
4 files changed, 61 insertions(+), 115 deletions(-)