nameSetElems can introduce nondeterminism and while I haven't
observed this being a problem in practice (possibly because this
is dead code) there's no downside to doing this.

Test Plan: ./validate

Reviewers: bgamari, austin, simonpj, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2296

GHC Trac Issues: #4012