Avoid superfluous UB checks in `IndexRange` `IndexRange::len` is justified as an overall invariant, and `take_prefix` and `take_suffix` are justified by local branch conditions. A few more UB-checked calls remain in cases that are only supported locally by `debug_assert!`, which won't do anything in distributed builds, so those UB checks may still be useful. We generally expect core's `#![rustc_preserve_ub_checks]` to optimize away in user's release builds, but the mere presence of that extra code can sometimes inhibit optimization, as seen in #131563.