Makes the AuxInt arg to Move/Zero print in a readable format.

Change-Id: I12295959b00ff7c1638d35836cc6d64d112c11ca
Reviewed-on: https://go-review.googlesource.com/28271
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>