Fix data accesses for simple loopfilters

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