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