Add a new garbage collector pass.

For identity like ops (a=b), we sometimes run into use-after-free and
double-free bugs.

For this snippet
    J100 = J99
    J101 = J99 + 3         <- last use of J99
    J102 = J100 * 2        <- last use of J100
before we were doing:
    J100 = J99
    J101 = J99 + 3
    free(J99)
    J102 = J100 * 2        <- use-after-free
    free(J100)             <- double-free
now we do:
    J100 = J99
    J101 = J99 + 3
    J102 = J100 * 2
    free(J100)

Algorithm:
We iterate through the program in reverse order and every time we see a
use of a variable, we insert a free after it, unless we have already
freed it before. When we check a variable has been freed, we also check
whether any of its aliases have also been freed.

For alias analysis, we maintain alias sets using disjoint sets. Whenever
we encounter an a=b statement, we simply do a union of a and b sets.

This replaces the old LivenessOpti pass.