David Feuer authored Mar 14, 2017 and David Feuer committed Mar 14, 2017

`exprIsOk` didn't shortcut properly when checking `case` (it used
`foldl` inappropriately). Fix that.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3343