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

[via llvm-commits]

vporpo wrote:

I made a design change: There are now two DAG Node classes: `DGNode` for non-mem instructions and `MemDGNode` for memory dependency candidates. A chain links `MemDGNode` together.

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