cmd/internal/obj/x86: Make VPSHUFD accept negative constant

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>

src/cmd/internal/obj/x86/asm6.go

@@ -857,6 +857,8 @@ var yvex_ri3 = []ytab{
 var yvex_xyi3 = []ytab{
 	{Yu8, Yxm, Yxr, Zvex_i_rm_r, 2},
 	{Yu8, Yym, Yyr, Zvex_i_rm_r, 2},
+	{Yi8, Yxm, Yxr, Zvex_i_rm_r, 2},
+	{Yi8, Yym, Yyr, Zvex_i_rm_r, 2},
 }
 
 var yvex_yyi4 = []ytab{ //TODO don't hide 4 op, some version have xmm version
@@ -1712,7 +1714,7 @@ var optab =
 	{AVPBROADCASTB, yvex_vpbroadcast, Pvex, [23]uint8{VEX_128_66_0F38_W0, 0x78, VEX_256_66_0F38_W0, 0x78}},
 	{AVPTEST, yvex_xy2, Pvex, [23]uint8{VEX_128_66_0F38_WIG, 0x17, VEX_256_66_0F38_WIG, 0x17}},
 	{AVPSHUFB, yvex_xy3, Pvex, [23]uint8{VEX_128_66_0F38_WIG, 0x00, VEX_256_66_0F38_WIG, 0x00}},
-	{AVPSHUFD, yvex_xyi3, Pvex, [23]uint8{VEX_128_66_0F_WIG, 0x70, VEX_256_66_0F_WIG, 0x70}},
+	{AVPSHUFD, yvex_xyi3, Pvex, [23]uint8{VEX_128_66_0F_WIG, 0x70, VEX_256_66_0F_WIG, 0x70, VEX_128_66_0F_WIG, 0x70, VEX_256_66_0F_WIG, 0x70}},
 	{AVPOR, yvex_xy3, Pvex, [23]uint8{VEX_128_66_0F_WIG, 0xeb, VEX_256_66_0F_WIG, 0xeb}},
 	{AVPADDQ, yvex_xy3, Pvex, [23]uint8{VEX_128_66_0F_WIG, 0xd4, VEX_256_66_0F_WIG, 0xd4}},
 	{AVPADDD, yvex_xy3, Pvex, [23]uint8{VEX_128_66_0F_WIG, 0xfe, VEX_256_66_0F_WIG, 0xfe}},