This simplifies the branch analysis we perform on the bytecode stream
down to the bare minimum that we need to build graphs. Note that we
still record all branch targets, even though only the backwards ones
would be needed, but this is essentially for free and might be useful
eventually.

R=oth@chromium.org

Review URL: https://codereview.chromium.org/1646873004

Cr-Commit-Position: refs/heads/master@{#33635}