This looks like a copy-paste mistake introduced in de2aca288569, that
has added checks for more patterns of SET statements while ignoring the
original test block that existed.
Backpatch down to where this has been introduced, as this shaves some
cycles.
Author: Sergei Kornilov
Discussion: https://postgr.es/m/5689421699428803@mail-sendbernar-production-main-46.myt.yp-c.yandex.net
Backpatch-through: 16