This is one cycle faster in total, and three instructions fewer.

Before:
vp9_loop_filter_mix2_v_44_16_neon: 123.2
After:
vp9_loop_filter_mix2_v_44_16_neon: 122.2

This is cherrypicked from libav commit
3bf9c483.
Signed-off-by: Martin Storsjö <martin@martin.st>