[llvm] [IR] PossiblyExactOperator -> PossiblyExactInst (NFC) (PR #72501)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle approved this pull request.

LGTM. I'm fine with the name change, thanks for keeping the alias for now.

I don't really have an opinion on the `setExact` implementation itself. As long as there's an `Instruction::setExact`, it may as well be implement there directly. If anything, I'd question whether `Instruction::setExact` should exist in the first place. I think there's a good argument that there should only be a `PossiblyExactInst::setExact` method -- though I expect making that change would require more changes elsewhere.

https://github.com/llvm/llvm-project/pull/72501