The free vars of `ty2` need to be in scope to satisfy the substitution
invariant.
As far as I can tell we don't have the free vars of `ty2` when
substituting, so unfortunately we have to compute them.

Test Plan: ./validate

Reviewers: austin, bgamari, simonpj, goldfire

Subscribers: thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D2024

GHC Trac Issues: #11371