AstGraphBuilder::BuildToBoolean() can be optimized easily without types,
especially since the types are only present on some nodes during graph
building. So this optimization is both more efficient and more effective
at the same time. We will probably refactor this code into a separate
optimization method/class later.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1188503003

Cr-Commit-Position: refs/heads/master@{#29062}