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