[llvm] [MachinePipeliner] Fix store-store dependences (#72508) (PR #72575)

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bcahoon wrote:

> Do you have any thoughts on how it should be modified? I think the following two modifications are needed.
> 
> * Fix `addLoopCarriedDependences()` to verify dependencies for all pairs of memory accesses where at least one is a store.
> * Fix the graph so it does not need to be a DAG and represent upward dependencies in a straightforward manner.

That's a good question. I think the near term solution is to change addLoopCarriedDependences as you describe to fix the correctness issue. Longer term is to create a graph with the dependences. I think that type of change will take some non-trivial effort.

https://github.com/llvm/llvm-project/pull/72575