Avoid unneeded nbtree backwards scan buffer locks.

Teach nbtree backwards scans to avoid relocking a just-read leaf page to
read its current left sibling link when it isn't truly necessary.  This
happened inside _bt_readnextpage whenever _bt_readpage had already
determined that there'll be no further matches to the left (or at least
none for the current primitive index scan, for a scan with array keys).

A new precheck inside _bt_readnextpage is all that we need to avoid
these useless lock acquisitions.  Arguably, using a precheck like this
was a missed opportunity for commit 2ed5b87f96, which taught nbtree to
drop leaf page pins early to avoid blocking cleanup by VACUUM.  Forwards
scans already managed to avoid relocking the page like this.

The optimization added by this commit is particularly helpful with
backwards scans that use array keys where the scan must perform multiple
primitive index scans.  Such backwards scans will now avoid a useless
leaf page re-lock at the end of each primitive index scan.

Note that this commit does not attempt to avoid needlessly re-locking a
leaf page that was just read when the scan must follow the leaf page's
left link.  That more ambitious optimization could work by stashing the
left link when the page is first read by a backwards scan, allowing the
subsequent _bt_readnextpage call to optimistically skip re-reading the
original page just to get a new copy of its left link.  For now we only
address cases where we don't care about our original page's left link.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wz=xgs7PojG=EUvhgadwENzu_mY_riNh-w9wFPsaS717ew@mail.gmail.com

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