[llvm] [MachinePipeliner] Fix incorrect handlings of unpipelineable insts (PR #126057)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> OK. I suspect that ignoring the artificial dependence is OK because it's just trying to force scheduling results that reduce copying, but I can't be 100% sure.

I also think that ignoring the artificial dependencies doesn't break anything. However, we should also consider that there are any other `Mutation`s, not only `CopyToPhiMutation`, appended by `getSMSMutations`. And they might add artificial dependencies for other reasons. I believe artificial dependencies should not affect correctness, but I'm not sure.
https://github.com/llvm/llvm-project/blob/387f3e8f986d53067a68aa0d7b058a0ce81ec941/llvm/include/llvm/CodeGen/MachinePipeliner.h#L330

>  it would be good for the comments in the code to expand a little more about what's going on, specifically mentioning that artificial dependences are preventing the "obvious" initial ordering from being used.

Added detailed comments.

https://github.com/llvm/llvm-project/pull/126057