[llvm] IR: introduce struct with CmpInst::Predicate and samesign (PR #116867)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

>     * Can you please move this out of CmpInst? The fact that CmpInst::Predicate cannot be forward-declared has been a regular source of friction -- we sometimes use `unsigned` in headers just because of this. We can avoid repeating that mistake now...

Thanks for the context.
 
>     * I'm not a big fan of the name `PredicateSign`. That sounds like "the sign of the predicate" not "predicate plus samesign"...

... and here I was thinking I was being clever and elegant migrating CmpInst::Predicate -> CmpInst::PredicateSign.

https://github.com/llvm/llvm-project/pull/116867