[llvm] [RISCV][MRI] Account for fixed registers when determining callee saved regs (PR #115756)

[Craig Topper via llvm-commits]

topperc wrote:

> The behavior differs from GCC for AArch64, ARM, Hexagon, SPARC, M68k and RISC-V.
> 
> In LLVM, different targets have different interfaces to check whether the register was `-ffixed-XXX`. I am proposing that each target migrate to this common interface in follow up patches.
> 
> This patch adds the interface, and migrates RISC-V and M68k to that interface, since they are already using the interface.

How will this interact with AArch64 or other targets calling setCalleeSavedRegisters which will cause `IsUpdatedCSRsInitialized` to get set preventing reaching the new code you added in `getCalleeSavedRegs`

https://github.com/llvm/llvm-project/pull/115756