We need the order of specialized binds and rules to be deterministic,
so we use a deterministic set here.

Test Plan: ./validate

Reviewers: simonmar, bgamari, austin, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2197

GHC Trac Issues: #4012