[llvm] [APFloat] Fix sign bit corrupting exponent in unsigned FP8 formats (PR #223286)

[Tim Gymnich via llvm-commits]

tgymnich wrote:

@jay87406 Before merging it would be great if you had a public email associated with your commits.

> Are any special GitHub settings required to contribute to LLVM?
>
>We only require contributors to have a public email address associated with their GitHub commits, see this [section](https://llvm.org/docs/DeveloperPolicy.html#email-addresses) of LLVM Developer Policy for details.

https://github.com/llvm/llvm-project/pull/223286