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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

I've updated the branch with the latest prototype. To try the branch, add the following flags to clang:
`-mllvm -riscv-enable-save-csr-in-ra=true `  
`mllvm -min-weight-ratio-needed-to-evict-hint=7.5` - this controls a new heuristic I added to register allocator. I've arrived at `7.5` by experiment, so other value could work better in your case.

Looking forward to your feedback. If there are no principal issues moving forward I can start upstreaming this work in small commits.

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