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

[Jay Foad via llvm-commits]

jayfoad wrote:

> The existing implemention has some buggy behavior: The scheduler does not reset the pointer to next cluster candidate. For example, we want to cluster A and B, but after picking A, we might pick node C. In theory, we should reset the next cluster candiate here, because we have decided not to cluster A and B during scheduling. Later picking B because of Cluster seems not logical.

Could you fix that bug first in a separate PR? Then this PR should have fewer test check changes.

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