Rect DC_PRED: mult and shifts equivalent to div.

(1) We use an initial variable shift, and then
(2) A multiply + 16 bit shift for bit-depth = 8, OR
    A multiply + 17 bit shift for bit-depth = 10 and 12

All the constants (shifts and multipliers) for different block sizes are
based on "Algorithm 1" in
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1467632

Note:
This also reverts test hashes etc to the version before
commit 37ebf187.

BUG=aomedia:1191

Change-Id: I2aaebc3e95958a3f145c0408aa5cbcc85e30c8dc