[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> > Any chance we could rebase this patch on main? It would enable me to run some more experiments. Currently having some trouble when I tried to rebase it myself.
> 
> @michaelmaitland I'll do it soon, sorry for delay. I had to switch to something else for now.

I rebased and pushed to a branch in my private repo, so we can updo the rebase if needed.

https://github.com/llvm/llvm-project/pull/90819