arm64: loopfilter16: Fix conditions for skipping parts of the filtering
arm64: loopfilter16: Fix conditions for skipping parts of the filtering

As the arm64 16 bpc loopfilter operates on a 8 pixel region at a time,
inspect 2 bits (corresponding to 4 pixels each) from these registers,
as we also shift them down by 2 bits at the end of the loop.

This should allow skipping the loopfilter altogether (or using a
smaller filter) in more cases.

videolan/dav1d