Bug fix: ssse3 version of subpixel did not match C code

A 16 bit overflow condition occurs when using the EIGHTTAP_SMOOTH filters.
(vp9_sub_pel_filters_8lp)  Changed the order of the adds to fix this problem.
Also added ssse3 support for 4x4 subpixel filtering.

Change-Id: I475eaadae920794c2de5e01e9735c059a856518e