[llvm] [MacroFusion] Early return when insts already clustered (PR #191710)

[Tomer Shafir via llvm-commits]

tomershafir wrote:

Can you provide more context for the question - why do we need single node clusters?

I think it should not affect scheduling in general, because machine scheduler only schedules at least 2 nodes in the same cluster together. So a single-node cluster should not change the behavior. 

For AArch64, I am not aware of target-specific logic that would violate this behavior.

https://github.com/llvm/llvm-project/pull/191710