Previously we first calculated hev, and then negated it.

Since we were able to schedule the negation in the middle
of another calculation, we don't see any gain in all cases.

Before:                     Cortex A7      A8      A9     A53  A53/AArch64
vp9_loop_filter_v_4_8_neon:     147.0   129.0   115.8    89.0         88.7
vp9_loop_filter_v_8_8_neon:     242.0   198.5   174.7   140.0        136.7
vp9_loop_filter_v_16_8_neon:    500.0   419.5   382.7   293.0        275.7
vp9_loop_filter_v_16_16_neon:   971.2   825.5   731.5   579.0        453.0
After:
vp9_loop_filter_v_4_8_neon:     143.0   127.7   114.8    88.0         87.7
vp9_loop_filter_v_8_8_neon:     241.0   197.2   173.7   140.0        136.7
vp9_loop_filter_v_16_8_neon:    497.0   419.5   379.7   293.0        275.7
vp9_loop_filter_v_16_16_neon:   965.2   818.7   731.4   579.0        452.0

This is cherrypicked from libav commit
e1f9de86.
Signed-off-by: Martin Storsjö <martin@martin.st>