This re-introduces the long lost concept of conditional builder methods
into the AstGraphBuilder that are allowed to return {nullptr} when a
certain optimization does not apply. This can be used to separate our
optimizations in the graph builder clearly from code required for
correctness.

R=bmeurer@chromium.org
BUG=v8:4513
LOG=n

Review URL: https://codereview.chromium.org/1414723004

Cr-Commit-Position: refs/heads/master@{#31694}