[llvm] [SandboxVec][DAG] Implement functions for iterating through DGNodes (PR #109684)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

You are upstreaming this because you have already experience with it and thought about the design.

Two questions: how does it relate to MemorySSA and graph traits?

https://github.com/llvm/llvm-project/pull/109684