[clang] [llvm] [InstCombine][CVP][SCCP] Add support for `uitofp nneg` (PR #86154)

[via cfe-commits]

goldsteinn wrote:

I looked into adding support in `SCEV` but it didn't seem worth it to
add complete support for `uitofp`/`sitofp` just for this. Any opinions
on if I should revisit / any other passes that I should add support
for? Mostly I just copied where we do the same for `zext`.

https://github.com/llvm/llvm-project/pull/86154