Although checkasm does not verify this, the decoder requires that the
transform updates the input block exactly like the C code does.

This fixes vc1-ism, vc1_ilaced_twomv, vc1_sa00040, vc1_sa10091,
vc1_sa10143, vc1_sa20021, vc1test_smm0005 and wmv3-drm-dec tests.