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/include/access/nbtree.h

@@ -1155,7 +1155,7 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
  */
 extern void _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel,
 						   IndexTuple newitem, Size newitemsz,
-						   bool checkingunique);
+						   bool bottomupdedup);
 extern bool _bt_bottomupdel_pass(Relation rel, Buffer buf, Relation heapRel,
 								 Size newitemsz);
 extern void _bt_dedup_start_pending(BTDedupState state, IndexTuple base,