[compiler-rt] [builtins] Start to refactor int to fp conversion functions to use a common implementation (PR #66903)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM - Note that I'm only LGTMing this because of the alive2 proof Alex mentioned above.  I wouldn't be comfortable approving this (correctness wise) otherwise.  Given we know it's correct, all that's left is the stylistic question and this looks entirely reasonable to me.  I'm definitely out of area though, so if someone more active in this area has an opinion, please speak up.  

https://github.com/llvm/llvm-project/pull/66903