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

[Quentin Colombet via llvm-commits]

================
@@ -317,6 +317,8 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
     return false;
   }
 
+  virtual bool doCSRSavesInRA() const;
----------------
qcolombet wrote:

Ditto missing comments

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