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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

Note that this branch does not have the LICM change. I think now there are more degradations than before. Going to look into that. But, I really like to work on merging this part first, so I do not have to rebase it constantly.

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