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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> What is the thought process of taking this approach over modifying the ShrinkWrap pass to communicate the PrologEpilogInserter where to put each callee saved register?

If you ignore the CFI stuff, this approach is much easier, simplifies things    (now we don't special treatment of CSRs in RA) and allows for more    optimizations than just shrink-wrapping by itself. Essentially, we're exposing  the code to save / restore CSRs to all the optimizations after ISel.

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