[llvm] [CycleInfo] Represent cycles by an opaque handle. NFC (PR #210117)

[Jay Foad via llvm-commits]

jayfoad wrote:

Sorry if this is a dumb question, I have not been following these patches too closely:

If cycles are now identified by CycleRef which wraps an integer, does that mean we no longer need pointer stability for the array of cycles, and does that mean we can stop using deque in GenericCycleImpl? (This is just my personal prejudice against deque -- I think it is almost never a good idea to use it.)

https://github.com/llvm/llvm-project/pull/210117