Implement branch prediction intrinsics

[nagisa]

Moved from rust-lang/rust#20579.

Branch prediction intrinsic equivalent to GCC’s __builtin_expect should exist. These correspond to the llvm.expect intrinsics. Alternatively two rust intrinsics likely and unlikely could be implemented.

See rust-lang/rust#17317, rust-lang/rust#22805 and rust-lang/rust#22883 as well.

[Stebalien]

Triage: Fixed by #1131

[qguv]

It looks like this was put on hold waiting for MIR. Has there been any progress now that MIR has landed?

[sinkuu]

RFC 1131 is already implemented and waiting for stabilization, right?

[qguv]

That's what I was looking for. Thanks @sinkuu!