pgsql: Split out tiebreaker comparisons from comparetup_* functions

Split out tiebreaker comparisons from comparetup_* functions

Previously, if a specialized comparator found equal datum1 keys,
the "comparetup" function would repeat the comparison on the
datum before proceeding with the unabbreviated first key
and/or additional sort keys.

Move comparing additional sort keys into "tiebreak" functions so
that specialized comparators can call these directly if needed,
avoiding duplicate work.

Reviewed by David Rowley

Discussion: https://postgr.es/m/CAFBsxsGaVfUrjTghpf%3DkDBYY%3DjWx1PN-fuusVe7Vw5s0XqGdGw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c9bfa40914be4a0882b904834c2bd32775a50d7b

Modified Files
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src/backend/utils/sort/tuplesort.c | 9 +--
src/backend/utils/sort/tuplesortvariants.c | 125 ++++++++++++++++++++++-------
src/include/utils/tuplesort.h | 7 ++
3 files changed, 108 insertions(+), 33 deletions(-)