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

[via llvm-commits]

vporpo wrote:

It's not relying on MemorySSA for now, but we may switch to it at some point.
Regarding GraphTraits, they are not really needed for now, but I will implement them at some point.

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