Optimizes the AVX128 blends by reusing the prior SSE4.1 implementation.
Only difference is the destination register isn't reused as a source
register.
One confusing thing is that Felix Cloutier's documentation has a typo on
the 256-bit VPBLENDW instruction where it had the top 128-bit lane
reusing the destination instead of sources. So I wrote a unittest to
ensure correctness.
Fixes#3796
Ryan Houdek
f8ef6feff9