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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

I should clarify the situation with branches / patches:

(1) There exist 2 POC branches:
the original one (this one)
and rebased on a more recent, but still old main: https://github.com/mgudim/llvm-project/tree/rebased_save_csr_in_ra
These have one huge commit and are impossible to understand, they are not for review, they are just proof-of-concept.

(2) I have another branch: https://github.com/mgudim/llvm-project/tree/save_csr_in_ra3 where I split up all the work from (1) into small commits. This branch is still in the broken state and it is missing 3 - 4 commits.

(3) As commits are ready in (2) and can be merged into main I am posting them as individual PRs. Here's an up-to-date list:
https://github.com/llvm/llvm-project/pull/168869
https://github.com/llvm/llvm-project/pull/168531
https://github.com/llvm/llvm-project/pull/166773
https://github.com/llvm/llvm-project/pull/166763
https://github.com/llvm/llvm-project/pull/164480

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