The only significant change here is that

   case e of {}

should be treated like 'e', rather than like a case expression.
We don't push a return address, for example, since 'e' is sure to
diverge.

I forget why I did this; but it will make these empty-case expressions
(which are only there to satisfy the type checker) cost-free.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3204