[llvm] [AMDGPU] Add DAG mutation to improve scheduling before barriers (PR #142716)

[Jay Foad via llvm-commits]

jayfoad wrote:

I'm trying to understand this. In some cases the load is required to complete before the fence for correctness, so there must already be an edge in DAG representing that, right? So why do you need to add extra edges? Are you handling cases where the ordering was _not_ required for correctness? Or do you just want to set a different latency on the edge? If so, can't you use `adjustSchedDependency` for that?

https://github.com/llvm/llvm-project/pull/142716