[llvm] [CycleInfo] Index using block numbers instead of pointers (PR #187500)

[Alexis Engelke via llvm-commits]

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

LGTM in general.

Unrelated, just fyi: making CycleInfo immutable is [rather easy right now](https://github.com/llvm/llvm-project/compare/97e1addcf57d2c7b11d2a6761e5be202e06eec1b...0015997ea8b10979f1245a0d53357ed03e1b94b5); I do want to continue to explore that path (locally) and see what we can gain from more compact data structures.

https://github.com/llvm/llvm-project/pull/187500