Fix "single value strategy" index deletion issue.

It is not appropriate for deduplication to apply single value strategy
when triggered by a bottom-up index deletion pass.  This wastes cycles
because later bottom-up deletion passes will overinterpret older
duplicate tuples that deduplication actually just skipped over "by
design".  It also makes bottom-up deletion much less effective for low
cardinality indexes that happen to cross a meaningless "index has single
key value per leaf page" threshold.

To fix, slightly narrow the conditions under which deduplication's
single value strategy is considered.  We already avoided the strategy
for a unique index, since our high level goal must just be to buy time
for VACUUM to run (not to buy space).  We'll now also avoid it when we
just had a bottom-up pass that reported failure.  The two cases share
the same high level goal, and already overlapped significantly, so this
approach is quite natural.

Oversight in commit d168b666, which added bottom-up index deletion.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznaOvM+Gyj-JQ0X=JxoMDxctDTYjiEuETdAGbF5EUc3MA@mail.gmail.com
Backpatch: 14-, where bottom-up deletion was introduced.

src/backend/access/nbtree/nbtinsert.c

@@ -2748,7 +2748,7 @@ _bt_delete_or_dedup_one_page(Relation rel, Relation heapRel,
 	/* Perform deduplication pass (when enabled and index-is-allequalimage) */
 	if (BTGetDeduplicateItems(rel) && itup_key->allequalimage)
 		_bt_dedup_pass(rel, buffer, heapRel, insertstate->itup,
-					   insertstate->itemsz, checkingunique);
+					   insertstate->itemsz, (indexUnchanged || uniquedup));
 }
 
 /*