aarch64: vp9: loop filter: replace 'orr; cbn?z' with 'adds; b.{eq,ne};

The latter is 1 cycle faster on a cortex-53 and since the operands are
bytewise (or larger) bitmask (impossible to overflow to zero) both are
equivalent.

This is cherrypicked from libav commit
e7ae8f7a.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>