Factor code from ABI_CallFunctionRR and GetWriteTrampoline into a helper, and fix a special case.

The special case is where the registers are actually to be swapped (i.e.
func(ABI_PARAM2, ABI_PARAM1); this was previously impossible but would
be ugly not to handle anyway.

Source/Core/Common/x64Emitter.h

@@ -753,6 +753,9 @@ public:
 	void ABI_CallFunctionR(void *func, X64Reg reg1);
 	void ABI_CallFunctionRR(void *func, X64Reg reg1, X64Reg reg2, bool noProlog = false);
 
+	// Helper method for the above, or can be used separately.
+	void MOVTwo(int bits, Gen::X64Reg dst1, Gen::X64Reg src1, Gen::X64Reg dst2, Gen::X64Reg src2, Gen::X64Reg temp);
+
 	// A function that doesn't have any control over what it will do to regs,
 	// such as the dispatcher, should be surrounded by these.
 	void ABI_PushAllCalleeSavedRegsAndAdjustStack();