Optimize ImmutableArray<T>.ElementAtOrDefault

[prozolic]

This PR optimizes the bounds checking logic for index validation in ImmutableArray<T>.ElementAtOrDefault.
The current implementation uses index < 0 || index >= immutableArray.Length, but I replaced this with unsigned comparison (uint)index >= (uint)immutableArray.Length to reduce bounds checks.
This pattern is already used in Enumerable.ElementAtOrDefault, making this change safe and consistent.

[stephentoub]

@EgorBo, the JIT is already successfully eliding the bounds check here based on the comparisons, but it's still ending up with two branches:
SharpLab
Is that expected?

[EgorBo]

Is that expected?

Yes, these have different semantics: if index is negative - one of them returns default, the other fails with NRE

[stephentoub]

Ah, right, because ImmutableArray.Length just returns array.Length, and array may be null in the case of default(ImmutableArray).

[stephentoub]

@prozolic, thanks, but as this would change behavior when consuming default(ImmutableArray), we can't take the PR. Appreciate the attempt, though.

[prozolic]

You're right, default(ImmutableArray) was an oversight on my part. Thank you for pointing that out.