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

[via llvm-commits]

ZERICO2005 wrote:

> > Poison values can create conflicting bits, asserting would crash the compiler. It's probably best to just check for conflicts and bail out with ~fcNone~ `Known.resetAll()`
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> It now returns `fcAll`* if there are any conflicting bits for the input.
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> *value from the default constructor of `KnownFPClass`

Would it still be best to return `fcAll` for **all** `APFloat` types?
If a type doesn't support infinity, but `KnownBits` has conflicts (poison), should we return `fcAll` or `fcAll & ~fcInf`? The other option is having poison return `fcNone` instead.

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