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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

Results: 

<img width="810" height="380" alt="image" src="https://github.com/user-attachments/assets/31397d00-e1c5-403a-8e12-d530d7a3b495" />


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