[llvm] [MachineScheduler] Make cluster check more efficient (PR #150884)

[via llvm-commits]

ruiling wrote:

> Can you confirm the compile-time gain?

I don't see the compile-time issue on my side. I am not sure whether @mysterymath can help on this as he observed the issue. But I think the change would help reduce compile time as it stops calling `SmallSet::contains()`.

https://github.com/llvm/llvm-project/pull/150884