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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

 When using a `Visited` list to stop infinite recursion, the list must be reverted to the previous state when leaving each level. Think of two `icmp` instructions referencing the same PHI (in reachable code!). The `evaluateOnPredecessorEdge` on the first icmp will mark the PHI as visited, the second would think the PHI is recursive since it was already visited.

Could it be sufficient to check `CondComp->getOperand(0) == CondComp` /  `CondComp->getOperand(1) == CondComp` at some point? If iteratively replacing values with identical values, it should evantually replace everything in the chain with the same value. Also reminds me of [finding a loop lin a singly-linked list](https://stackoverflow.com/questions/10275587/finding-loop-in-a-singly-linked-list#).



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