enable idct4x4* and idct8x8* which are compatible for 8-bit decodes in
high-bitdepth mode. the adapter narrows 32-bit input to 16, whether the
expansion can be avoided at all in this case remains a TODO. roughly
matches sse2.
BUG=webm:1294
Change-Id: I3ea94e5a2070dfd509b5de0c555aab4e1f4da036