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

[Alex Bradbury via llvm-commits]

asb wrote:

@mgudim @ppenzin I was interested in kicking the tires on this to see impact on recentish HEAD on SPEC. Do either of you have a recent version of this patchset that is known good? I built https://github.com/mgudim/llvm-project/tree/save_csr_in_ra3 but I'm getting compiler crashes at the initial cmake configure for llvm-test-suite with `-mllvm -riscv-save-csrs-early=true`, or when I do it with `-save-temps=obj` as well I get errors about unrecognized instruction mnemonic `BUNDLE`.

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