Implement branch prediction intrinsics

[nagisa]

Branch prediction intrinsic equivalent to GCC’s __builtin_expect should exist. Alternatively two intrinsics likely and unlikely could be implemented.

[nagisa]

Appears to correspond to llvm.expect intrinsic.

[jdm]

See #17317.

[nagisa]

I’m be fine with only expect_u* exposed from core::intrinsics. That wouldn’t need any RFC I think. While likely! and unlikely! are nice-to-have, they are simple enough to define in terms of the intrinsics.

[alexcrichton]

cc @steveklabnik, could this move to the rust-lang/rfcs repo? (feature request)