This change isn't necessary for determinism. appAName, choiceAName,
loopAName all have pre-allocated Uniques and their relative order
can't change. I opted to use nameSetElemsStable here because:
* the cost is negligible
* it's less fragile than just documenting

Test Plan: ./validate

Reviewers: simonpj, austin, bgamari, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2292

GHC Trac Issues: #4012