[llvm] ValueTracking: Identify implied fp classes by general fcmp (PR #66505)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > @akuegel remember this test failure in TF last week? maybe you have additional information here?
> 
> I have debugged now which class is detected, and it is `FPClassTest::fcPosInf`. Does that help? 

Not really, I want a test case which reproduces the failure. I don't want to spend unbounded time guessing on what fell out of this test case that managed hit this 

> 
> ```
> if (RHSClass == fcPosZero || RHSClass == fcNegZero)
>     return {nullptr, fcAllFlags, fcAllFlags};
> ```
> 
> Any reason why fcPosInf, fcNegInf, etc. is not just treated in the same way? It seems the original assert was written with some assumption that doesn't hold. I also verified that changing it to:

Infinities are simpler than 0 as they do not depend on the denormal mode. This will be a trivial fix if I can just get a reproducer 

https://github.com/llvm/llvm-project/pull/66505