x86: faster AVX satd/sa8d/sa8d_satd/hadamard_ac

Use Conroe-style movddup in AVX transforms; both Sandy Bridge and Bulldozer
do movddup in the load unit, so it's totally free this way.
~5-6% faster hadamard_ac and sa8d_satd. Doesn't seem to help satd/sa8d as much.