x86: Add intial VEX support.

Support some 2-op 256-bit AVX instructions.
Namely VMOVNTDQ, VMOVNTDQA, VMOVDQA, VMOVDQU, VZEROUPPER.

Change-Id: I16225d3aeaba628c1dd64ce8bfe032a7f8a8c846
Reviewed-on: https://go-review.googlesource.com/14126
Reviewed-by: Russ Cox <rsc@golang.org>
8 files changed