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

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM. What are hot users of df_iterator?

The loop vectorizer, I noticed this when looking at `vp_depth_first_shallow` etc. which are used quite frequently. 

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