This has to be prevented because inlining would result in multiple
self-references, which we don't support (and in fact that's disallowed
by the SQL spec, see statements about linearly vs. nonlinearly
recursive queries).  Bug fix for commit 608b167f.

Per report from Yaroslav Schekin (via Andrew Gierth)

Discussion: https://postgr.es/m/87wolmg60q.fsf@news-spur.riddles.org.uk