In subpixel filters, prefetched source data, unrolled loops,
and interleaved instructions.

In HORIZx4, integrated the idea in Scott's CL (commit:
d22a504d), which was suggested by
Erik/Tamar from Intel. Further tweaking was done to combine row 0,
2, and row 1, 3 in registers to do more 2-row-in-1 operations until
the last add.

Test showed a ~2% decoder speedup.

Change-Id: Ib53d04ede8166c38c3dc744da8c6f737ce26a0e3