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

[Nikita Popov via cfe-commits]

nikic wrote:

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

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