Using pg_leftmost_one_post32() yields substantial performance benefits.

Backpatching to version 13 because HLL is used for HashAgg
improvements in 9878b643, which was also backpatched to 13.

Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzkGvDKVDo+0YvfvZ+1CE=iCi88DCOGFF3i1hTGGaxcKPw@mail.gmail.com
Backpatch-through: 13