fc555cd832
Also get `add nuw` from `uN::checked_add`
When I was doing this for `checked_{sub,shl,shr}`, it was mentioned https://github.com/rust-lang/rust/pull/124114#issuecomment-2066173305 that it'd be worth trying for `checked_add` too.
It makes a particularly-big difference for `x.checked_add(C)`, as doing this means that LLVM removes the intrinsic and does it as a normal `x <= MAX - C` instead.
cc `@DianQK` who had commented about `checked_add` related to https://github.com/rust-lang/hashbrown/issues/509 before
cc https://github.com/llvm/llvm-project/issues/80637 for how LLVM is unlikely to do this itself