Summary:
Commit b4bdbe49 (the fix for #12156)
wound up being the fix for #12947 and #13640 as well. This adds regression
tests for the latter two tickets to keep them fixed.

Test Plan: make test TEST="T12947 T13640"

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #12947, #13640

Differential Revision: https://phabricator.haskell.org/D3528