[llvm] [IR][DAG] Add support for `nneg` flag with `uitofp` (PR #86141)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Realize still have logic in `SelectionDAGBuilder`. Do you want me to split that as well?

That's the codegen part, so yes? 

https://github.com/llvm/llvm-project/pull/86141