reuse common loopfilter code

there were four versions for the regular and
macroblock loopfilters:
horizontal [y|uv]
vertical [y|uv]

this moves all the common code into 2 functions:
vp8_loop_filter_neon
vp8_mbloop_filter_neon

this provides no gain in performance. there's a bit
of jitter, but it trends down ~0.25-0.5%. however,
this is a huge gain maintenance. also, there is the
potential to drop some stack usage in the macroblock
loopfilter.

Change-Id: I91506f07d2f449631ff67ad6f1b3f3be63b81a92