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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> Will this still work with -msave-restore and cm.push/pop?

I think so. All it does is it makes a callee-saved register a spill, so this should work. I'll run spec with these options to check. If we need to optimize for size, than we can just turn this off with a flag.

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