expose a way to mark cold/hot branches with llvm.expect

[thestinger]

There are places where this would be very useful like the slow path in a vector push method. Sadly, there's not an obvious way to expose llvm.expect because a regular Rust intrinsic becomes a function and lower-expect runs in the function pass stage.

[jdm]

See #8291 where llvm.expect was turned down.

[thestinger]

@jdm: I've seen it, but I still think this is a useful feature. It's true that there are only rare use cases where it's a good idea, and I already covered most failure cases by marking the fail_ lang item as a cold.

[thestinger]

This can actually already be done with #[link_name].