gen_prune_steps_from_opexps's notion of how to do this was overly
complicated and underly correct.

Per discussion of a report from Alan Jackson (though this fixes only one
aspect of that problem).  Back-patch to v11 where this code came in.

Amit Langote

Discussion: https://postgr.es/m/FAD28A83-AC73-489E-A058-2681FA31D648@tvsquared.com