Split vp8/vp9 implementations on yv12_copy_frame_c.
Remove high-bitdepth codes from vp8_yv12_extend_frame_borders_c.
Clean up vp8 codes usage in vp9.

BUG=webm:1435

Change-Id: Ic68e79e9d71e1b20ddfc451fb8dcf2447861236d