Use backwards propagation for liveness analyses on tags.

Use backwards propagation for liveness analyses on tags.

In commit a7a26d17cee825afcbe8659cb31a04a165172eed we added fix-point
iterative algorithm to do liveness analyses on tags:

    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.

However, instead of iterating until fix point is reached, one may
use backwards propagation of live variables starting from the points
where these variables are used.