After detecting a sub-match "dissect" failure (i.e., a backref match
failure) in the i'th sub-match of an iteration node, we should proceed
by adjusting the attempted length of the i'th submatch.  As coded,
though, these functions changed the attempted length of the *last*
sub-match, and only after exhausting all possibilities for that would
they back up to adjust the next-to-last sub-match, and then the
second-from-last, etc; all of which is wasted effort, since only
changing the start or length of the i'th sub-match can possibly make
it succeed.  This oversight creates the possibility for exponentially
bad performance.  Fortunately the problem is masked in most cases by
optimizations or constraints applied elsewhere; which explains why
we'd not noticed it before.  But it is possible to reach the problem
with fairly simple, if contrived, regexps.

Oversight in my commit 173e29aa.  That's pretty ancient now,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/1808998.1629412269@sss.pgh.pa.us