Summary:
Some recent commits happened to fix other issues:

* c2417b87 fixed #13601 and #13780
* 8e15e3d3 fixed the original program in #13877

Let's add regression tests for each of these to ensure they stay fixed.

Test Plan: make test TEST="T13601 T13780a T13780c T13877"

Reviewers: goldfire, bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13601, #13780, #13877

Differential Revision: https://phabricator.haskell.org/D3794