I had to refactor some things to take VarSet instead of [Var],
but I think it's more precise this way.

Test Plan: ./validate

Reviewers: simonmar, simonpj, austin, bgamari, goldfire

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2227

GHC Trac Issues: #4012