[llvm] [CycleInfo] Identify cycles with a single-pass DFS algorithm (PR #210491)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. The last changes improved perf by another ~5%. We should remove the Entries vector from Cycle afterwards, though. Please wait for a few days before merging to give others a chance to chime in.

Thank you for implementing this and also the previous improvements to CycleInfo.

https://github.com/llvm/llvm-project/pull/210491