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

[via llvm-commits]

https://github.com/ruiling approved this pull request.

Code change LGTM.

A side question: does it work with your target architecture if we continue adding the un-clustered node into the parent cluster group if we see only one node being clustered?

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