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

[Quentin Colombet via llvm-commits]

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@@ -210,6 +211,11 @@ class TargetFrameLowering {
   /// for noreturn nounwind functions.
   virtual bool enableCalleeSaveSkip(const MachineFunction &MF) const;
 
+  virtual void emitCFIsForCSRsHandledByRA(MachineFunction &MF,
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qcolombet wrote:

We need a comment here to tell backend authors what they are supposed to do with this hook and when they should consider implementing it.

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