[llvm] [FixIrreducible] Use CycleInfo instead of a custom SCC traversal (PR #101386)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

Note that I have not yet finished verifying all the lit tests. I might also have to add a few more tests, especially involving a mix of irreducible and reducible cycles that are siblings and/or nested inside each other in various combinations. Especially with some overlap in the entry and header nodes.

https://github.com/llvm/llvm-project/pull/101386