Encode SSE instructions as AVX

[dmitri.veprinsky@…]

In order to do easy porting from SSE to AVX. What do you think about feature, that generate AVX code by SSE instructions For example

addpd xmm1, xmm2 should be encoded as vaddpd xmm1,xmm1,xmm2 addpd xmm1, <mem> should be encoded as vaddpd xmm1,xmm1,<mem>

[peter@…]

We support this already. Just add a "v" prefix, so vaddpd xmm1, xmm2 -> vaddpd xmm1, xmm1, xmm2.

I considered adding a "DEFAULT VEX" directive to change addpd to generate the VEX version, but in discussion with the NASM folks this idea was shot down.