aarch64: vp9: use alternative returns in the core loop filter function

Since aarch64 has enough free general purpose registers use them to
branch to the appropiate storage code. 1-2 cycles faster for the
functions using loop_filter 8/16, ... on a cortex-a53. Mixed results
(up to 2 cycles faster/slower) on a cortex-a57.

This is cherrypicked from libav commit
d7595de0.

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