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

[via llvm-commits]

dpenry wrote:

The description of the problem doesn't say enough about what was going wrong to form any opinion of the correctness of this patch.  Nor does the debug message give enough information to even confirm that there was a problem. 

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