Avoid X86_ASM ifdef.
Ideally, we may want to update all routines to average vertically
first, which would make this unnecessary. In the interim, this
enables the tests to run successfully on x86 without SSSE3 support
again.
Average vertically before horizontally; horizontal averaging is more
worksome. Doing the vertical averaging first reduces the number of
horizontal averages by half.
Use pmaddubsw and pavgw to do the horizontal averaging for a slight
performance improvement.
Minor tweaks.
Improve the SSSE3 dyadic downsample routines and drop the SSE4 routines.
The non-temporal loads used in the SSE4 routines do nothing for cache-
backed memory AFAIK.
Adjust tests because averaging vertically first gives slightly different
output.
~2.39x speedup for the widthx32 routine on Haswell when not memory-bound.
~2.20x speedup for the widthx16 routine on Haswell when not memory-bound.
Note that the widthx16 routine can be unrolled for further speedup.
