"MULBB R1, R2, R3" is encoded to 0xe163f182, which should be
0xe1630182.

This patch fix it.

fix #20764

Change-Id: I9d3c3ffa40ecde86638e5e083eacc67578caebf4
Reviewed-on: https://go-review.googlesource.com/46491
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>