LDADDALD(64-bit) and LDADDALW(32-bit) are already supported.
This CL adds supports of LDADDALH(16-bit) and LDADDALB(8-bit).

Change-Id: I4eac61adcec226d618dfce88618a2b98f5f1afe7
Reviewed-on: https://go-review.googlesource.com/132135
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>