This makes the call to `substTy` satisfy the invariant from
Note [The substitution invariant] in TyCoRep.

Test Plan: ./validate --slow

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1861

GHC Trac Issues: #11371