Rewrite HORIZx4 and HORIZx8 in subpixel filter functions

Rewrite HORIZx4 and HORIZx8 in subpixel filter functions

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

In HORIZx4, integrated the idea in Scott's CL (commit:
d22a504d11a15dc3eab666859db0046b5a7d75c5), 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