* Calculate sx4, sy4 by truncation instead of rounding
* Move some repeated calculations out of the filter loop

This is expected to have a roughly neutral effect on BDRATE.
The speedup of each filter (SSE2, lowbd SSSE3, highbd SSSE3) is
7-10%, for a total speedup of 14-18% when considered together
with patches f7a5ee53 and 14b8112b.

Change-Id: I692f649202214c7ab53ecf81f81386f1503e2d20