Only used in 4:2:2. Both 8-bit and high bit-depth implemented.

Approximate performance improvement compared to C on Ivy Bridge:

                         x86-32  x86-64
idct_dequant_2x4_dc      2.1x    1.7x
idct_dequant_2x4_dconly  2.7x    2.0x

Helps more on 32-bit due to the C versions being register starved.