[llvm] [KnownFPClass] Refine known classes for `KnownFPClass::bitcast` (PR #215708)

[via llvm-commits]

ZERICO2005 wrote:

Sure I can make it return `fcAll`. I still have to check `Bits.hasConflict()` otherwise a non-deterministic value (that could be neither `fcAll` nor `fcNone`) may be returned.

https://github.com/llvm/llvm-project/pull/215708