cmd/internal/obj/x86: Make VPSHUFD accept negative constant
This partially reverts commit
4e24e1d9996b0b0155c8349e49244d9694c89708.
Since in release 1.7 VPSHUFD support negative constant as an argument,
removing it as part of
4e24e1d9996b0b0155c8349e49244d9694c89708 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>