[llvm] [AArch64] Mark GPR64arg register class as non-allocatable (PR #212457)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

This looks OK. Is the regclass only used in this code?
```
bool AArch64RegisterInfo::isAnyArgRegReserved(const MachineFunction &MF) const {
  for (size_t i = 0; i < AArch64::GPR64argRegClass.getNumRegs(); ++i) {
    if (MF.getSubtarget<AArch64Subtarget>().isXRegisterReserved(i))
      return true;
  }
  return false;
}
```

So we construct a register class with 8 regs, to check that the size is 8? Could we remove it and just check against 8?

https://github.com/llvm/llvm-project/pull/212457