This is the only call site of `lhs_cmp_type` and we only
care about equality.
`cmpType` is nondeterministic (because `TyCon`s are compared
with Uniques in `cmpTc`), so if we don't have to use it, it's
better not to.

Test Plan: ./validate

Reviewers: simonmar, goldfire, bgamari, austin, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2172

GHC Trac Issues: #4012