[llvm] [AMDGPU] misched: avoid subregister dependencies (PR #140255)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

@arsenm I think the instructions unpacked by https://github.com/llvm/llvm-project/pull/157968 in `llvm/test/CodeGen/AMDGPU/llvm.amdgcn.iglp.opt.exp.large.mir` do not get unpacked after my changes because they are no longer dependencies. But ig it does make sense to not let them be dependencies in the first place, right?

https://github.com/llvm/llvm-project/pull/140255