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

[Craig Topper via llvm-commits]

topperc 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.

I think you should continue.

I'm also curious how this interacts with -fno-omit-frame-pointer which makes the X8 register reserved, but its still in the callee save list.

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