6l: add MOVQ xmm_reg, xmm_reg

Added handler for:
        MOVQ xmm_reg, xmm_reg/mem64
        MOVQ xmm_reg/mem64, xmm_reg
using native MOVQ (it take precedence above REX.W MOVD)
I don't understood 6l code enough to be sure that my small changes
didn't broke it. But now 6l works with MOVQ xmm_reg, xmm_reg and
all.bash reports "0 unexpected bugs".

There is test assembly source:
MOVQ    X0, X1
MOVQ    AX, X1
MOVQ    X1, AX
MOVQ    xxx+8(FP), X2
MOVQ    X2, xxx+8(FP)

and generated code (gdb disassemble /r):

0x000000000040f112 <+0>:   f3 0f 7e c8        movq  %xmm0,%xmm1
0x000000000040f116 <+4>:   66 48 0f 6e c8     movq  %rax,%xmm1
0x000000000040f11b <+9>:   66 48 0f 7e c8     movq  %xmm1,%rax
0x000000000040f120 <+14>:  f3 0f 7e 54 24 10  movq  0x10(%rsp),%xmm2
0x000000000040f126 <+20>:  66 0f d6 54 24 10  movq  %xmm2,0x10(%rsp)

Fixes #2418.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5316076

src/cmd/6l/l.h

@@ -163,7 +163,7 @@ struct	Optab
 	short	as;
 	uchar*	ytab;
 	uchar	prefix;
-	uchar	op[20];
+	uchar	op[22];
 };
 struct	Movtab
 {

src/cmd/6l/optab.c

@@ -200,7 +200,8 @@ uchar	ymovq[] =
 	Ymm,	Ymr,	Zm_r_xm,	1,	// MMX MOVD
 	Ymr,	Ymm,	Zr_m_xm,	1,	// MMX MOVD
 	Yxr,	Ymr,	Zm_r_xm_nr,	2,	// MOVDQ2Q
-	Yxr,	Ym,	Zr_m_xm_nr,	2,	// MOVQ xmm store
+	Yxm,	Yxr,	Zm_r_xm_nr,	2, // MOVQ xmm1/m64 -> xmm2
+	Yxr,	Yxm,	Zr_m_xm_nr,	2, // MOVQ xmm1 -> xmm2/m64
 	Yml,	Yxr,	Zm_r_xm,	2,	// MOVD xmm load
 	Yxr,	Yml,	Zr_m_xm,	2,	// MOVD xmm store
 	Yiauto,	Yrl,	Zaut_r,	2,	// built-in LEAQ
@@ -862,7 +863,7 @@ Optab optab[] =
 	{ AMOVNTPD,	yxr_ml,	Pe, 0x2b },
 	{ AMOVNTPS,	yxr_ml,	Pm, 0x2b },
 	{ AMOVNTQ,	ymr_ml,	Pm, 0xe7 },
-	{ AMOVQ,	ymovq,	Pw, 0x89,0x8b,0x31,0xc7,(00),0xb8,0xc7,(00),0x6f,0x7f,0x6e,0x7e,Pf2,0xd6,Pe,0xd6,Pe,0x6e,Pe,0x7e },
+	{ AMOVQ,	ymovq,	Pw, 0x89, 0x8b, 0x31, 0xc7,(00), 0xb8, 0xc7,(00), 0x6f, 0x7f, 0x6e, 0x7e, Pf2,0xd6, Pf3,0x7e, Pe,0xd6, Pe,0x6e, Pe,0x7e },
 	{ AMOVQOZX,	ymrxr,	Pf3, 0xd6,0x7e },
 	{ AMOVSB,	ynone,	Pb, 0xa4 },
 	{ AMOVSD,	yxmov,	Pf2, 0x10,0x11 },

src/cmd/6l/span.c

@@ -266,10 +266,6 @@ instinit(void)
 	ycover[Ym*Ymax + Ymm] = 1;
 	ycover[Ymr*Ymax + Ymm] = 1;
 
-	ycover[Yax*Ymax + Yxm] = 1;
-	ycover[Ycx*Ymax + Yxm] = 1;
-	ycover[Yrx*Ymax + Yxm] = 1;
-	ycover[Yrl*Ymax + Yxm] = 1;
 	ycover[Ym*Ymax + Yxm] = 1;
 	ycover[Yxr*Ymax + Yxm] = 1;