[llvm] [BFI] Simplify irreducible header discovery. NFC (PR #213213)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Plan to improve BlockFrequencyInfo ( Wu-Larus algorithm with irreducible loop extension)

- [BFI] Simplify irreducible header discovery. NFC
- Then BFI on CycleInfo. Should fix the CodeGen include.
- Then delete IrreducibleSubgraph. Wu-Larus algorithm cannot handle a reducible loop with an irreducible subset, `@loop_around_irreducible` in `llvm/test/Analysis/BlockFrequencyInfo/irreducible.ll`)


https://github.com/llvm/llvm-project/pull/213213