libvpx

Fix data accesses for simple loopfilters

The data that the simple horizontal loopfilter reads is aligned, treat
it accordingly.

For the vertical, we only use the bottom 4 bytes, so don't read in 16
(and incur the penalty for unaligned access).

This shows a small improvement on older processors which have a
significant penalty for unaligned reads.

postproc_mmx.c is unused

Change-Id: I87b29bbc0c3b19ee1ca1de3c4f47332a53087b3d