Use pmovmskb to skip quantize loops over empty coefficients.

Use pmovmskb to skip quantize loops over empty coefficients.

If none of the 16 coefficients that we quantize per loop iteration
are larger than the zbin, directly skip to the next round of coeffs,
rather than doing a full quantize loop that will eventually result
in 16 zeroes. This incurs a jump cost, but saves a lot of other work.
32x32 quant goes from 1349 -> 1184 cycles. The same approach yielded
no significantly positive results for smaller transforms, so is not
used there (8x8: 103 -> 101 cycles; 16x16: 302 -> 306 cycles).

Change-Id: I8fca17dc2543fc8eed1dbcd5100145e3c3a9b647