attempt to make register allocation deterministic

examples/sum/sum.s

@@ -3,16 +3,16 @@
 
 // func Sum(xs []uint64) uint64
 TEXT ·Sum(SB),0,$0-32
-	MOVQ	xs_base(FP), DX
-	MOVQ	xs_len+8(FP), AX
-	XORQ	CX, CX
+	MOVQ	xs_base(FP), AX
+	MOVQ	xs_len+8(FP), CX
+	XORQ	DX, DX
 loop:
-	CMPQ	AX, $0x0
+	CMPQ	CX, $0x0
 	JE	done
-	ADDQ	(DX), CX
-	ADDQ	$0x8, DX
-	DECQ	AX
+	ADDQ	(AX), DX
+	ADDQ	$0x8, AX
+	DECQ	CX
 	JMP	loop
 done:
-	MOVQ	CX, ret+24(FP)
+	MOVQ	DX, ret+24(FP)
 	RET