[llvm] [MachineScheduler] Turn SU->isScheduled check into an assert in pickNode() (PR #160145)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM. I think we will always pick from these SUs that haven't been scheduled. 

https://github.com/llvm/llvm-project/pull/160145