[llvm] Rebased save csr in ra (PR #131845)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland commented:

Thanks for posting this. It **really** helps to see the full picture here because the additional context is **necessary** in giving review.

I'd really like to see you split this into a series of patches, all stacked on top of eachother. That way, we can review in small chunks but refer to the stacked components for context.

Please see #119359 for an example. The stacking of PRs makes the job as a reviewer much easier, and it will allow me to give feedback faster.

I left some comments here. Some are questions I hope you can answer here, and some are comments I hope can be useful as you split into stacked patches. Lastly, I think some of the changes in this PR have already made it in (like the reaching def stuff)

https://github.com/llvm/llvm-project/pull/131845