[libc][NFC] refactor fmin and fmax (#86718)

Hello,

So, I worked on the fmaximum and fminimum functions recently and the
reviewers suggested the structure:

```
if (bitsx ...)
  return ...;
if (bitsy ..)
  return 
...
return ...;
```
So I went ahead and did the same for fmin and fmax. I hope this isnt an
issue for you all. thanks.

---------

Co-authored-by: Job Hernandez <h93@protonmail.com>