Handle typeof applied to empty in InstSimplify

Summary:
Do not simplify `typeof` if it is applied to an invalid type. This
handles a case like the one in the added test, where `typeof` is called
on a literal empty in unreachable code.

Reviewed By: kodafb

Differential Revision: D31000173

fbshipit-source-id: 2d7f69cbcc9c1bb0a916585c07171089444c85dc