The only way `readMutVar#` or `writeMutVar#` can fail is if its
argument is not a valid pointer. I believe we ensure this by
construction, and never test for pointer validity. So I think it
should be safe to say that it can't fail.

Fixes #13424

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3340