[llvm] [IR][JumpThreading] Fix infinite recursion on compare self-reference [updated] (PR #129501)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

After talking about this again with @Meinersbur , I think I got a better view on this issue.
I initially didn't really understand your comment [here](https://github.com/llvm/llvm-project/pull/129501#pullrequestreview-2655373280), @nikic . Sorry about that. I was a bit confused because the pass itself has this comment about requiring the removal of unreachable blocks to prevent invalid IR. So I thought that we must either prevent invalid IR or remove unreachable blocks properly. But I now see that it's ok to just handle invalid IR and pass it along (it will then fail at a later stage).

https://github.com/llvm/llvm-project/pull/129501