[llvm] [ValueTracking] Fix computeKnownFPClass handling of nsz (PR #186315)

[Yunbo Ni via llvm-commits]

cardigan1008 wrote:

> Hm... shouldn't we handle nsz generically somehow? If nsz is set, then for all inputs we need to treat treat a pzero known FP class as also including nzero and vice versa.

Thanks for the suggestion. I've updated the fix to widen the zero with nsz set. 

https://github.com/llvm/llvm-project/pull/186315