Use Hacker's Delight impl in `i64::midpoint` instead of wide `i128` impl
This PR switches `i64::midpoint` and (`isize::midpoint` where `isize == i64`) to using our Hacker's Delight impl instead of wide `i128` implementation.
As LLVM seems to be outperformed by the complexity of signed 128-bits number compared to our Hacker's Delight implementation.[^1]
It doesn't seems like it's an improvement for the other sizes[^2], so we let them with the wide implementation.
[^1]: https://rust.godbolt.org/z/ravE75EYj
[^2]: https://rust.godbolt.org/z/fzr171zKh
r? libs
bors
16422dbd89