pgsql: Rename find_em_expr_usable_for_sorting_rel.

Rename find_em_expr_usable_for_sorting_rel.

I didn't particularly like this function name, as it fails to
express what's going on. Also, returning the sort expression
alone isn't too helpful --- typically, a caller would also
need some other fields of the EquivalenceMember. But the
sole caller really only needs a bool result, so let's make
it "bool relation_can_be_sorted_early()".

Discussion: /messages/by-id/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7645376774c8532159f5f0f905e5e734d4ccbb18

Modified Files
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src/backend/optimizer/path/allpaths.c | 21 ++++++++++-----------
src/backend/optimizer/path/equivclass.c | 27 +++++++++++++++------------
src/include/optimizer/paths.h | 7 +++----
3 files changed, 28 insertions(+), 27 deletions(-)