[llvm] [llvm] Support multiple save/restore points in mir (PR #119357)

[Philip Reames via llvm-commits]

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

LGTM

For context of other reviewers....

This patch is not perfect (see below), but it's a reasonable step and has been stuck in review for an extremely long time.  I am personally committing to following up on a few of the nits so that this can make forward progress.

A couple points for the record:
* This currently only models the case where *all* CSRs are spill/filled at the same set of points.  This was a deliberate simplification for this patch, and will likely be revisited in future patches.
* This currently parses both the old MIR syntax and the new array syntax.  This was triggered by an old review comment by me, and I think everyone agrees this was a bad idea in terms of net complexity.  Once this lands, I'll post a patch which removes the variant bits, and updates the existing MIR tests in one go.  

Once this goes in, we can move forward with reviewing the actual changes to shrink wrapping.  

https://github.com/llvm/llvm-project/pull/119357