[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
> 
> Then I don't get what you meant by "such that the value is always directly usable without checking the returned value".

I mean fcAllFlags is the correct conservative value to use for the unknown case. fcNone is extremely aggressive 

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