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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> > As previously requested, I've taken the contents of @mgudim's branch and made a PR stack:
> > 
> > * PR 1/5: [[WIP][CodeGen][RISCV] Allow multiple location for the same CSR. #170606](https://github.com/llvm/llvm-project/pull/170606)
> > * PR 2/5: [[WIP][CodeGen][DebugInfo][RISCV] Support scalable offsets in CFI #170607](https://github.com/llvm/llvm-project/pull/170607)
> > * PR 3/5: [[WIP][CodeGen] Split `determineCalleeSaves` #170609](https://github.com/llvm/llvm-project/pull/170609)
> > * PR 4/5: [[WIP][CodeGen] Enable early CFI #170610](https://github.com/llvm/llvm-project/pull/170610)
> > * PR 5/5: [[WIP][CodeGen] Allow register allocator to drive save/restore #170611](https://github.com/llvm/llvm-project/pull/170611)
> > 
> > I've omitted one "fix a test' commit and a couple of commits are swapped by accident. It is also rebased on a more recent `main`. PR 2 has clean CI and can be probably extracted out, though might require additional DWARF verification.
> 
> @ppenzin @asb I already have prs posted for several commits in this branch

@ppenzin 
I already have PRs open for all commits that can be merged now. Can you please close your PRs because this makes the situation confusing. Also, I would like to remain the author of my commits.

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