[opus] AVX Optimizations

[Timothy B. Terriberry]

Velea, Radu wrote:
> In the meantime if you have time to give it a quick look and maybe point out any errors or problems at this stage I'd greatly appreciate it.

I took a brief look and didn't see any obvious bugs, but we will want 
you to organize the commits so that each one is independently compilable 
and testable (and doesn't break anything) before you submit them. I 
understand what you've got up now is just a WIP, though.

I'll try to go over them more thoroughly in the next few days.