AMDGPU: Start defining a calling convention

Partially implement callee-side for arguments and return values.
byval doesn't work properly, and most likely sret or other on-stack
return values most as well.

llvm-svn: 303308
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.h b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.h
index 09bdf8f..251cb7a 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.h
@@ -38,7 +38,8 @@
                    unsigned VReg) const override;
   bool lowerFormalArguments(MachineIRBuilder &MIRBuilder, const Function &F,
                             ArrayRef<unsigned> VRegs) const override;
-  CCAssignFn *CCAssignFnForCall(CallingConv::ID CC, bool IsVarArg) const;
+  static CCAssignFn *CCAssignFnForCall(CallingConv::ID CC, bool IsVarArg);
+  static CCAssignFn *CCAssignFnForReturn(CallingConv::ID CC, bool IsVarArg);
 };
 } // End of namespace llvm;
 #endif