[llvm] ConstraintElim: teach fact-transfer about samesign (PR #115893)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Probably makes sense to land #116867 first and then use it here...

I realized that it's also better to first land #118534.

https://github.com/llvm/llvm-project/pull/115893