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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Well I still don't get what kind of caller could use the returned mask without also checking the returned Value. An example might help.

I'm not actually doing this for the benefit of the caller, I'm trying to avoid confusion in the implementation itself 

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