Add an expect intrinsic to the stdlib.

[mahkoh]

extern {
    #[link_name = "llvm.expect.i8"]
    fn expect(val: u8, expected_val: u8) -> u8;
}

[pnkfelix]

For those not famililar with LLVM, here's the spec for what this does:

• http://llvm.org/docs/LangRef.html#llvm-expect-intrinsic

(i.e. its a hint to LLVM that the expression passed in for val is likely to be the value expected_val.)

[dotdash]

We're already using that internally, just needs to be exposed (does that need an RFC?). The relevant places are declare_intrinsics(), get_simple_intrinsic() and the extern "rust-intrinsic" block in src/libcore/intrinsics.rs.

[steveklabnik]

(does that need an RFC?)

The rules are very... in-flux now, but adding new public things will soon, yes. I almost closed this and suggested the RFCs repo, but I'm leaving it for now since it's gray area.

[nagisa]

Duplicate of #20579. Also see #17317