[llvm] ValueTracking: Use fcAllFlags for unknown value (PR #66393)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Do you have an example of how this would simplify some caller?

This just sort of ends up happening in my attempt to merge fcmpToClassTest and fcmpImpliesClass after #65983

https://github.com/llvm/llvm-project/pull/66393