[llvm] [ADT] Use SmallVector in df_iterator. NFC (PR #225787)

[Alexis Engelke via llvm-commits]

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

LGTM. What are hot users of df_iterator?

(Aside: df_iterator should, ideally, be refactored into a structure like PostOrderTraversal to not store the state in the iterators, no longer use this strange iterator storage scheme, and support using block numbers. Unfortunately, refactoring df_iterator is rather difficult as it is used rather often in cases where single iterators are returned instead of ranges...)

https://github.com/llvm/llvm-project/pull/225787