Unary `!` on unused awaitables is dangerous

Summary: Extend "dangerous awaitable" check to unary not.

Reviewed By: @pikatchu

Differential Revision: D1383362
Initial work to get loops through the JIT

Summary: This diff tweaks the hot-trace region former to expose some loops
(under a runtime option), and does a few necessary changes to get the
IR built and going through the backend.  All optimization passes are
disabled for now, and only register allocation and code generation are
performed.

Reviewed By: @bertmaher

Differential Revision: D1391337