[llvm] [KnownFPClass] Combine fpext and fptrunc into a shared routine (PR #225188)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

To my understanding, the only real/practical difference between `fptrunc` and `fpext` is preserving NaN payloads.

I feel like it might be better to merge the logic for `fptrunc` and `fpext` since `KnownFPClass` does not deal with NaN payloads.

https://github.com/llvm/llvm-project/pull/225188