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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

I have rebased main optimization part and put it in (yet another new) branch: https://github.com/mgudim/llvm-project/tree/save-csrs-early

Since this doesn't handle CFIs yet,  I had more flags to disable CFI emission. So, the complete set of flags is:

```
 -mcpu=veyron-v1 \
 -fno-exceptions \
 -fno-unwind-tables \
 -fno-asynchronous-unwind-tables \
 -O3 \
  -mllvm -riscv-save-csrs-early \
-mllvm -min-weight-ratio-needed-to-evict-hint=7.5 \
```

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