Auto merge of #124114 - scottmcm:better-checked, r=workingjubilee

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

Author: bors