[llvm] [APInt][NFC] Add branch hints to APInt single-word (PR #219437)

[Steffen Larsen via llvm-commits]

steffenlarsen wrote:

Performance numbers can be found [here](https://llvm-compile-time-tracker.com/compare.php?from=52774473867e49b5891ab9381accf4e5a3ce0024&to=eeab9be613790199fe999b160138e684d56677cd&stat=instructions%3Au).

https://github.com/llvm/llvm-project/pull/219437