[llvm] [IR] Use block numbers in PredIteratorCache (PR #101885)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Someone skeptical about this one. It means that PredIteratorCache should not be used in cases where only a small fraction of blocks will be queried.

The change to store ArrayRef instead of having two separate maps looks good to me though.

https://github.com/llvm/llvm-project/pull/101885