Improve vp8_sixtap_predict functions

Restructure vp8_sixtap_predict functions to eliminate extra 5-line
calculation while doing first-pass only. Also, combline functions
to eliminate usage of intermediate buffer. This gives decoder a 3%
performance gain on my test clips.

Change-Id: I13de49638884d1a57d0855c63aea719316d08c1b