[llvm] [llvm][CodeGen] Add a new software pipeliner 'Window Scheduler' (PR #84443)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > How do you suggest we handle it now?
> 
> Just keep it as is. Please remember to do it next time :)

I see it squashed in the repo, so what is the issue? 

https://github.com/llvm/llvm-project/pull/84443