[llvm] [NewGVN] Prevent cyclic dependencies by ensuring Leader has min RPO number (PR #82110)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

The buildbot complains about 2 failing tests:
 - Transforms/NewGVN/br-identical.ll
 - Transforms/NewGVN/completeness.ll

https://github.com/llvm/llvm-project/pull/82110