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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

I support to continue to investigate where we can reach via this approach, I think this can be a good workaroud to enhance "shrink wrapping". Thanks for your great idea and I will do some evaluations too.

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