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

[Mikhail Gudim via llvm-commits]

mgudim 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`.

@asb I think this branch:
https://github.com/mgudim/llvm-project/tree/rebased_save_csr_in_ra
should be working. To enable saving csrs early don't forget to add these two flags:

```
-mllvm -riscv-enable-save-csr-in-ra=true 
-mllvm -min-weight-ratio-needed-to-evict-hint=7.5
```

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

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