See #15019. When removing an unnecessary type equality check in #14737,
several regression tests failed. The cause was that some coercions that
are actually Refl coercions weren't passed in as such, which made the
equality check needlessly complex (Refl coercions can be discarded in
this particular check immediately, without inspecting the types at all).

We fix that, and get additional performance improvements for free.

Reviewers: goldfire, bgamari, simonpj

Reviewed By: bgamari, simonpj

Subscribers: simonpj, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4635