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/SIFrameLowering.h b/llvm/lib/Target/AMDGPU/SIFrameLowering.h
index 7ccd02b..e17adbe 100644
--- a/llvm/lib/Target/AMDGPU/SIFrameLowering.h
+++ b/llvm/lib/Target/AMDGPU/SIFrameLowering.h
@@ -26,6 +26,8 @@
     AMDGPUFrameLowering(D, StackAl, LAO, TransAl) {}
   ~SIFrameLowering() override = default;
 
+  void emitEntryFunctionPrologue(MachineFunction &MF,
+                                 MachineBasicBlock &MBB) const;
   void emitPrologue(MachineFunction &MF,
                     MachineBasicBlock &MBB) const override;
   void emitEpilogue(MachineFunction &MF,