niteria authored Mar 29, 2016 and Ben Gamari committed Mar 29, 2016

We will use `ty` in the range of the substitution, hence
the substitution needs `ty`'s free vars in-scope.
They don't seem easily available by other means, so we
just compute them.

Test Plan: ./validate

Reviewers: austin, goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D2043

GHC Trac Issues: #11371