For this case, with 8 inputs but only changing 4 of them, we can fit
all 16 input pixels into a q register, and still have enough temporary
registers for doing the loop filter.

The wd=8 filters would require too many temporary registers for
processing all 16 pixels at once though.

Before:                          Cortex A7      A8     A9     A53
vp9_loop_filter_mix2_v_44_16_neon:   289.7   256.2  237.5   181.2
After:
vp9_loop_filter_mix2_v_44_16_neon:   221.2   150.5  177.7   138.0

This is cherrypicked from libav commit
575e31e9.
Signed-off-by: Martin Storsjö <martin@martin.st>