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

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dpenry 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 think I'm good with just refusing to pipeline here for now, but it would be good for the comment to expand a little more about what's going on, specifically mentioning that artificial dependences are preventing the "obvious" initial ordering from being used.

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