This simplifies how the BytecodeGraphBuilder simulates control flow by
reversing the propagation direction to forwards propagation. This is the
same direction as the data flow which is also a forward propagation. In
this way the analysis information needed at merge points is drastically
reduced while still retaining the same simulation power.

In short: We push down environments instead of pulling them.

R=oth@chromium.org

Review URL: https://codereview.chromium.org/1641893004

Cr-Commit-Position: refs/heads/master@{#33633}