Minor cleanup using BooleanConstant in graph builder.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/687103004

Cr-Commit-Position: refs/heads/master@{#24996}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24996 ce2b1a6d-e550-0410-aec6-3dcde31c8c00