MOVB $1, (AX) was being disassembled as MOVL $1, (AX).

Use the memory size to override the standard size.
Fix the tests.

Fixes #15922

Change-Id: If92fe74c33a21e5427c8c5cc97dd15e087edb860
Reviewed-on: https://go-review.googlesource.com/23608Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>