Fix space leaks in simplifier (Trac #13426)
The Join points commit (8d5cf8bf) introduced a space leak
somewhere in the simplifier. The extra strictness added in this commit
fixes the leak. Unfortunately I don't really understand the details.
Unfortunately, the extra strictness appears to result in more overall
allocations in some cases, even while the peak heap size decreases in others.
Test Plan: harbormaster
Reviewers: austin, bgamari
Reviewed By: bgamari
Differential Revision: https://phabricator.haskell.org/D3399