[llvm] MachineScheduler: Improve instruction clustering (PR #137784)

[Jay Foad via llvm-commits]

jayfoad wrote:

> The existing way of managing clustered nodes was done through adding weak edges between the neighbouring cluster nodes, which is a sort of ordered queue. And this will be later recorded as `NextClusterPred` or `NextClusterSucc` in `ScheduleDAGMI`.

Even after this patch, we still add weak "cluster" edges to the DAG. Is that still necessary?

https://github.com/llvm/llvm-project/pull/137784