Make `checked` ops emit *unchecked* LLVM operations where feasible For things with easily pre-checked overflow conditions -- shifts and unsigned subtraction -- write the checked methods in such a way that we stop emitting wrapping versions of them. For example, today <https://rust.godbolt.org/z/qM9YK8Txb> neither ```rust a.checked_sub(b).unwrap() ``` nor ```rust a.checked_sub(b).unwrap_unchecked() ``` actually optimizes to `sub nuw`. After this PR they do. cc #103299