[IR][JumpThreading] Fix infinite recursion on compare self-reference (#129501)
In unreachable code, constant PHI nodes may appear and be replaced by their
single value. As a result, instructions may become self-referencing. This
commit adds checks to avoid going into infinite recursion when handling
self-referencing compare instructions in `evaluateOnPredecessorEdge()`.
This LLVM defect was identified via the AMD Fuzzing project.