varSetElems introduces unnecessary non-determinism and while
I didn't estabilish experimentally that this matters here
I'm convinced that it will, because I expect pattern synonyms
to end up in interface files.

Test Plan: ./validate

Reviewers: austin, simonmar, bgamari, mpickering, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2206

GHC Trac Issues: #4012