SSE4.1 optimized chorba

SSE4.1 optimized chorba

This is ~25-30% faster than the SSE2 variant on a core2 quad. The main reason
for this has to do with the fact that, while incurring far fewer shifts,
an entirely separate stack buffer has to be managed that is the size of
the L1 cache on most CPUs. This was one of the main reasons the 32k
specialized function was slower for the scalar counterpart, despite auto
vectorizing. The auto vectorized loop was setting up the stack buffer at
unaligned offsets, which is detrimental to performance pre-nehalem.
Additionally, we were losing a fair bit of time to the zero
initialization, which we are now doing more selectively.

There are a ton of loads and stores happening, and for sure we are bound
on the fill buffer + store forwarding. An SSE2 version of this code is
probably possible by simply replacing the shifts with unpacks with zero
and the palignr's with shufpd's. I'm just not sure it'll be all that worth
it, though. We are gating against SSE4.1 not because we are using specifically
a 4.1 instruction but because that marks when Wolfdale came out and palignr
became a lot faster.