[llvm] [MachinePipeliner] Add an abstract layer to manipulate Data Dependenc… (PR #109918)

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bcahoon wrote:

Thanks for adding this change. I'm in favor of adding the abstraction as it is a big improvement over the existing way of using the DAG. I'm sure there will be changes or other improvements, but it's a step in the right direction. I've only superficially gone through the changes, so I defer to @dpenry, who seems good with the patch.

https://github.com/llvm/llvm-project/pull/109918