Skip redundant tokenization in rd loop

Skip redundant tokenization in rd loop

This commit makes the encoder skip the redundant tokenization process
in the rate-distortion optimization search loop, while updating the
entropy contexts accordingly. It makes the speed 0 encoding process
about 0.5% faster at no performance change.

Change-Id: I34a4155a0b5332afeb45c93a51c7f35a294d685c