Expand unconstrained nodes in pack_mb_tokens and loop on zeros.

Reduces Linux perf estimated cycle count for pack_mb_tokens on a
lossless encode on my desktop from 61858501855 to 48154040219 or from
26% of the overall profile to 21%.

Change-Id: I9ca3426d7e3272bc7f7030abda4f0d0cec87fb4a