Also split superblock handling code out of decode_macroblock() into
a new function decode_superblock(), for easier readability.

Derf +0.05%, HD +0.2%, STDHD +0.1%. We can likely get further gains
by allowing to select mb_skip_coeff for a subset of the complete SB
or something along those lines, because although this change allows
coding smaller transforms for bigger predictors, it increases the
overhead of coding EOBs to skip the parts where the residual is
near-zero, and thus the overall gain is not as high as we'd expect.

Change-Id: I552ce1286487267f504e3090b683e15515791efa