AMD64's ADDQconstmodify/ADDLconstmodify have similar logic with
other constmodify like operators, but seperated case statements.
This CL simplify them with a fallthrough.
Change-Id: Ia73ffeaddc5080182f68c06c9d9b48fe32a14e38
Reviewed-on: https://go-review.googlesource.com/135855
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>