cmd/compile: mark duffzero as using X0, not AX

duffzero was changed to use X0 instead of AX in
CL 14408.  This was missed as part of that change.

Change-Id: I72fb0114cfbc035b83bfaa8631d27e6740da2652
Reviewed-on: https://go-review.googlesource.com/16717
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/compile/internal/amd64/prog.go

@@ -226,7 +226,7 @@ var progtable = [x86.ALAST]obj.ProgInfo{
 	x86.ASTOSL:    {Flags: gc.OK, Reguse: AX | DI, Regset: DI},
 	x86.ASTOSQ:    {Flags: gc.OK, Reguse: AX | DI, Regset: DI},
 	x86.ASTOSW:    {Flags: gc.OK, Reguse: AX | DI, Regset: DI},
-	obj.ADUFFZERO: {Flags: gc.OK, Reguse: AX | DI, Regset: DI},
+	obj.ADUFFZERO: {Flags: gc.OK, Reguse: X0 | DI, Regset: DI},
 	x86.ASUBB:     {Flags: gc.SizeB | gc.LeftRead | RightRdwr | gc.SetCarry},
 	x86.ASUBL:     {Flags: gc.SizeL | gc.LeftRead | RightRdwr | gc.SetCarry},
 	x86.ASUBQ:     {Flags: gc.SizeQ | gc.LeftRead | RightRdwr | gc.SetCarry},