This partially reverts commit 4e24e1d9.
Since in release 1.7 VPSHUFD support negative constant as an argument,
removing it as part of 4e24e1d9 was wrong.
Add it back.

Change-Id: Id1a3e062fe8fb4cf538edb3f9970f0664f3f545f
Reviewed-on: https://go-review.googlesource.com/27712
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>