Auto merge of #112724 - scottmcm:simpler-unchecked-shifts, r=Mark-Simulacrum

[libs] Simplify `unchecked_{shl,shr}`

There's no need for the `const_eval_select` dance here.  And while I originally wrote the `.try_into().unwrap_unchecked()` implementation here, it's kinda a mess in MIR -- this new one is substantially simpler, as shown by the old one being above the inlining threshold but the new one being below it in the `mir-opt/inline/unchecked_shifts` tests.

We don't need `u32::checked_shl` doing a dance through both `Result` *and* `Option` 🙃

Author: bors