btree: downlink right separator/HIKEY optimization

(now really to -hackers)
Hi,

Over at [0] I'd implemented an optimization that allows us to skip
calling _bt_compare in _bt_moveright in many common cases. This patch,
when stacked on top of the prefix truncation patch, improves INSERT
performance by an additional 2-9%pt, with an extreme case of 45% in
the worscase index tests at [0].

The optimization is that we now recognze that our page split algorithm
all but guarantees that the HIKEY matches this page's downlink's right
separator key bytewise, excluding the data stored in the
IndexTupleData struct.

By caching the right separator index tuple in _bt_search, we can
compare the downlink's right separator and the HIKEY, and when they
are equal (memcmp() == 0) we don't have to call _bt_compare - the
HIKEY is known to be larger than the scan key, because our key is
smaller than the right separator, and thus transitively also smaller
than the HIKEY because it contains the same data. As _bt_compare can
call expensive user-provided functions, this can be a large
performance boon, especially when there are only a small number of
column getting compared on each page (e.g. index tuples of many 100s
of bytes, or dynamic prefix truncation is enabled).

By adding this, the number of _bt_compare calls per _bt_search is
often reduced by one per btree level.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)

PS. Best served with dynamic prefix truncation [1] and btree specialization [0].

[0] https://www.postgresql.org/message-id/CAEze2WiqOONRQTUT1p_ZV19nyMA69UU2s0e2dp+jSBM=j8snuw@mail.gmail.com
[1] https://www.postgresql.org/message-id/flat/CAEze2Wh-h20DmPSMXp4qHR0-ykh9=Z3ejX8MSsbikbOqaYe_OQ@mail.gmail.com