[llvm] [Support][KnownFPClass] Refine fdiv and fdiv_self class propagation + some minor fixes for propagateXorSign (PR #215014)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw requested changes to this pull request.

A correctness issue was identified by [llvm-hackme](https://github.com/dtcxzyw/llvm-hackme). Please see the [issue comment](https://github.com/llvm/llvm-project/pull/215014#issuecomment-5228346447) for details.

https://github.com/llvm/llvm-project/pull/215014