This is not the most precise test, unfortunately, but it does
demonstrate a modest improvement in compiler residency as a
result of the specializer don't-loop patch. A rather less
realistic variation on this has somewhat more dramatic effects.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3656