Ensure unreachable branch is eliminated

[EFanZh]

With #2704, I introduced two unwrap() calls in Select::poll, but I found that with opt-level=z, the compiler won’t be able to optimize these unwrap() calls, so maybe changing them to unwrap_unchecked() is a better idea? See https://godbolt.org/z/cef118zKe for comparison with opt-level=z.

Also, since unwrap_unchecked() is introduced in Rust 1.58.0, I have to implement this function manually.

[taiki-e]

Hmm, given that opt-level=s works, I feel this is like a compiler bug. https://godbolt.org/z/sPTdMo1Kd

[seanmonstar]

Unless the performance gains are critical, I'd tend to err on keeping the safer version, and hoping that the compiler can be made smart enough eventually.

[EFanZh]

It seems that the #[inline] attribute on Option::unwrap is not enough for the compiler to apply inlining, which causes the compiler not being able to do the optimization. And if I implement unwrap manually (panic instead of UB) with #[inline(always)], the compiler is able to do the optimization: https://godbolt.org/z/55savPbaE. Is this change acceptable?

[taiki-e]

Thanks! The new approach looks good to me. (It is enough simple and has no unsafe code)