This commit changes the SSSE3 assembly functions for idct32x32 to
support highbitdepth build.

On test clip fdJc1_IBKJA.248.webm, this cuts the speed difference
between hbd and lbd build from between 3-4% to 1-2%.

Change-Id: Ic3390e0113bc1ca5bba8ec80d1795ad31b484fca