[clang] [llvm] [InstCombine] Add canonicalization of `sitofp` -> `uitofp nneg` (PR #88299)

[via cfe-commits]

goldsteinn wrote:

> Can the implementation of foldFBinOpOfIntCastsFromSign be simplified to use nneg instead of KnownBits after this change?

yeah we could. Should I do a survey of existing folds first to ensure we don't incorrectly keep flags (like with `trunc nuw/nsw`) before integrating too deeply?

https://github.com/llvm/llvm-project/pull/88299