Fix index deletion latestRemovedXid bug.

Fix index deletion latestRemovedXid bug.

The logic for determining the latest removed XID for the purposes of
generating recovery conflicts in REDO routines was subtly broken.  It
failed to follow links from HOT chains, and so failed to consider all
relevant heap tuple headers in some cases.

To fix, expand the loop that deals with LP_REDIRECT line pointers to
also deal with HOT chains.  The new version of the loop is loosely based
on a similar loop from heap_prune_chain().

The impact of this bug is probably quite limited, since the horizon code
necessarily deals with heap tuples that are pointed to by LP_DEAD-set
index tuples.  The process of setting LP_DEAD index tuples (e.g. within
the kill_prior_tuple mechanism) is highly correlated with opportunistic
pruning of pointed-to heap tuples.  Plus the question of generating a
recovery conflict usually comes up some time after index tuple LP_DEAD
bits were initially set, unlike heap pruning, where a latestRemovedXid
is generated at the point of the pruning operation (heap pruning has no
deferred "would-be page split" style processing that produces conflicts
lazily).

Only backpatch to Postgres 12, the first version where this logic runs
during original execution (following commit 558a9165e08).  The index
latestRemovedXid mechanism has had the same bug since it first appeared
over 10 years ago (in commit a760893d), but backpatching to all
supported versions now seems like a bad idea on balance.  Running the
new improved code during recovery seems risky, especially given the lack
of complaints from the field.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wz=Eib393+HHcERK_9MtgNS7Ew1HY=RDC_g6GL46zM5C6Q@mail.gmail.com
Backpatch: 12-