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

[Michael Maitland via llvm-commits]

michaelmaitland 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.

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