[FEATURE REQUEST] Location-based working set for fixpoints

[lucaneg]

Description
Sorting nodes to process w.r.t. their location will reduce the time needed to compute fixpoints:

while (x > 0)
    x--;
f(x);

After the while guard is processed both, x-- and f(x) are added to the ws. x-- is analyzed first, and the while guard is placed back into the ws at the beginning. The analysis of f(x) will be deferred until the whole loop stabilizes, and will thus be analyzed only once.