PATCH: Add vzeroupper optimization for AVX

[Andi Kleen]

"H.J. Lu" <hongjiu.lu@intel.com> writes:
>
> At RTL expansion time, the vzeroupper optimization generates a
> vzeroupper_nop before function call and functin return if 256bit AVX
> instructions are used. The vzeroupper pass is run before final pass.
> It scans all reachable blocks:
>
> 1. Remove vzeroupper_nop when:
>     a. The upper 128bits of AVX regiters are known dead.

Is dead the correct term here? It would seem to need "unused" 

-Andi