[llvm-branch-commits] [CycleInfo] Number cycles and move the exit-block cache to GenericCycleInfo. NFC (PR #209666)

[Fangrui Song via llvm-branch-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/209666

Assign each cycle a preorder ID in layoutBlocks's Euler-tour walk and
move the caches into a lazily allocated table indexed by ID, shrinking
sizeof(GenericCycle) from 128 to 80. appendEntry/setSingleEntry no
longer invalidate: the exit-block set does not depend on entries. Delete
the unused GenericCycle::clear.

Aided by Claude Fable 5