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

[Petr Penzin via llvm-commits]

ppenzin wrote:

@preames @arsenm any unaddressed feedback? There is a note about merging this PR and the next one in series (because it introduces registers into tests), but IMO reducing scope of the review is probably preferable.

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