CREATE INDEX: do update index stats if autovacuum=off.

CREATE INDEX: do update index stats if autovacuum=off.

This fixes a thinko from commit d611f8b15.  The intent was to prevent
updating the stats of the pre-existing heap if autovacuum is off,
but it also disabled updating the stats of the just-created index.
There is AFAICS no good reason to do the latter, since there could not
be any pre-existing stats to refrain from overwriting, and the zeroed
stats that are there to begin with are very unlikely to be useful.
Moreover, the change broke our cross-version upgrade tests again.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1116282.1741374848@sss.pgh.pa.us