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

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

Current status: This is going to take a "bit" longer. Turns out that `CreateControlFlowHub` is a heavy hammer which redirects all edges from `Incoming` to `Outgoing`. But this destroys inner cycles when used to fix an irreducible cycle. For some cycle `C` with a subcycle `S`, a block `B` in `S` may have edges going to the header `H` of `C` as well as the header `E` of `S`. Now if the header `E` is also an entry of `C`, then the use of `CreateControlFlowHub` removes the edge from `B` to `E`, thus destroying `S`.

This needs a finer method that redirects only specific edges. Either that, or we let the pass destroy some cycles. But updating `CycleInfo` for these missing subcycles may be a fair amount of work too, so I would rather do it the right way.

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