[llvm] [ValueTracking] Compute KnownFP state from recursive select/phi. (PR #113686)

[David Green via llvm-commits]

davemgreen wrote:

> I think this approach is also applicable to `computeKnownBits`. I'll post a patch later :)

Nice, thanks! I had seen known bits had some handling of recurrences already, they didn't necessarily include selects.
I think we can expand this to phi+phi for the fpclass case too relatively easily, which will help in more complex cases.

I rebased over 577c7dd7cc4c5a9f62f9654cfa30ee9d55709426, which added a phase-ordering test.

https://github.com/llvm/llvm-project/pull/113686