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

[Petr Penzin via llvm-commits]

ppenzin wrote:

> That branch only has one huge commit and its' just POC. I am still working on breaking it up into smaller commits and putting them on (https://github.com/mgudim/llvm-project/tree/save_csr_in_ra3). But that branch is in broken state right now

That would explain the situation with the PR stack. I thought the branch was ready to upstream, didn't check the comment before working on it.

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