Test MO_MulMayOflo

Here we add a test exercising MO_MulMayOflo, addresssing #2 (closed).

Until now we made no attempt to test this machop since the backend is free to implement it conservatively, precluding treating it as we do most other non-callish machops. I briefly considered generalizing the reference implementation to allow sets of allowed results but this seemed to add more complexity than this case warrants and would make it harder to reason about the correctness of the testsuite.

Here I instead added a separate test which tests MO_MulMayOflo independently.

As GHC currently does not have surface syntax for this primop, this depends upon ghc!8331 (closed).

In service of fixing ghc#21624 (closed).