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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> > > Any chance we could rebase this patch on main? It would enable me to run some more experiments. Currently having some trouble when I tried to rebase it myself.
> > 
> > 
> > @michaelmaitland I'll do it soon, sorry for delay. I had to switch to something else for now.
> 
> I rebased and pushed to a branch in my fork (https://github.com/mgudim/llvm-project/tree/rebased_save_csr_in_ra), so we can updo the rebase if needed.

In my original patch there was a hack related to emission of CFI's and the CFI inserter. I am trying to get rid of that we can get rid of the RISCV CFI inserter.

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