[llvm] [SCCP] get rid of potentially dangling iterator (PR #105609)

[Alexandros Lamprineas via llvm-commits]

labrinea wrote:

> Ah, you are right ... but I still don't really see a reason to store it as a iterator, rather than just the two pointers.
In my opinion having to keep two pointers synchronized may be more error-prone (one may update one and forget the other).

https://github.com/llvm/llvm-project/pull/105609