Otherwise, set_plan_refs() can get applied to the same list
multiple times through different references, leading to chaos.

Amit Langote, Dilip Kumar, and Robert Haas, reviewed by Ashutosh
Bapat.  Original report by Sveinn Sveinsson.

Discussion: http://postgr.es/m/20170517141151.1435.79890@wrigleys.postgresql.org