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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

At this point I just want to understand if I should continue with this approach or not. If "yes", I'll investigate the degradation on xz, figure out what goes wrong if function unwinds and add tests.

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