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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Values might not just be reused by different icmp instructions, but also by different arguments and/or using different paths:
```llvm
Preheader:
  br label %LoopHeader

LoopHeader:
  %common_phi = phi i32 [%Preheader, 0], [%Latch, 1]
  br label %Latch

Latch:
  %phi1 = phi i32 [%LoopHeader, %common_phi]
  %phi2 = phi i32 [%LoopHeader, %common_phi]
  %cmp = icmp eq %phi1, %phi2
  br %cmp, label %LoopHeader, label %Exit

Exit:
```

Assuming that `evaluateOnPredecessorEdge` recurses into all of the PHIs (I dont know the exact exit conditions), on the second visit, `%common_phi` has already been marked as visited. This is reachable code, so this should not happen.

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