If it works, then we won't be storing two copies of all the tuples
that were just moved.  If not, VACUUM will still take care of it
eventually.  Per a report from AP and analysis from Amit Kapila, it
seems that a bulk load can cause splits fast enough that VACUUM won't
deal with the problem in time to prevent bloat.

Amit Kapila; I rewrote the comment.

Discussion: http://postgr.es/m/20170704105728.mwb72jebfmok2nm2@zip.com.au