[llvm] [KnownFPClass] Rule out non-zero finite results for fmul and fdiv (PR #231052)

[Eric Ross via llvm-commits]

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ZERICO2005 wrote:

Most of the `fdiv` cases were handled in this PR https://github.com/llvm/llvm-project/pull/214912

https://github.com/llvm/llvm-project/pull/231052