Optimize Neon implementation of svt_aom_lpf_6

The Neon implementations of svt_aom_lpf_vertical_6 and
svt_aom_lpf_horizontal_6 compute both filter4() and filter6() before
selecting for each element which filter is actually needed. In practice,
however, a lot of cases only need one of the filters, so specialize for
these scenarios, computing only the filters that are needed and
eliminate bitwise select. This makes the case where all filters are
needed slightly slower, but as it is far from the most common case this
is ok.

Also move the actual filter computation to separate functions to avoid
code duplication.