Fixed bug in tag liveness analyses (tags lost in loops).

Fixed bug in tag liveness analyses (tags lost in loops).

Example that revealed failure (added as test):
    (@p "ac")* "b" { @p }

We used forward propagation: starting from initial state, walk DFA
in deep-first order; in each state, add tags associated with rule
or fallback set if necessary, then recurse to child states, then
merge all child sets into this node's live set.

This algorithm cannot propagate live tags in loops in cases when
DFS visits loopbacks before non-looping paths (tags don't propagate
into loopback states).

The usual algorithm for liveness analyses doesn't use DFS;
instead, it iterates on DFA states until fix point is reached
(next iteration adds no changes to live set compared to the
previous iteration). The fix applies this algorithm.

A more efficient algorithm may be possible; one that uses backwards
propagation instead of fix point (I'm not sure).