Fix test_uint32_shr failing on debug builds.

[defuse]

This makes shr behave differently than Rust's >> in release mode but IMO how >> works in release mode is kinda dumb -- I could see myself assuming the argument to shr or the right argument to >> saturates instead of taking the value mod 32, and introducing security bugs that way -- so this seems safer.

[str4d]

This PR would need to be migrated over to the main rust crates.