Improve SSE2 slide hash performance

Improve SSE2 slide hash performance

At least on pre-nehalem CPUs, we get a > 50% improvement. This is
mostly due to the fact that we're opportunistically doing aligned loads
instead of unaligned loads.  This is something that is very likely to be
possible, given that the deflate stream initialization uses the zalloc
function, which most libraries don't override.  Our allocator aligns to
64 byte boundaries, meaning we can do aligned loads on even AVX512 for
the zstream->prev and zstream->head pointers. However, only pre-nehalem
CPUs _actually_ benefit from explicitly aligned load instructions.

The other thing being done here is we're unrolling the loop by a factor
of 2 so that we can get a tiny bit more ILP.  This improved performance
by another 5%-7% gain.