Commit 2b74bd9d did wonders for the
program reported in #12545. Let's add a perf test for it to make sure it
stays fast.

Test Plan: make test TEST=T12545

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #12545

Differential Revision: https://phabricator.haskell.org/D3632