Make use of CreateBoolNode in traversers This is just refactoring to clean up some duplicate code. A new test is added to make sure this doesn't break UnfoldShortCircuitAST. BUG=angleproject:1490 TEST=angle_unittests Change-Id: I108e646f71ba631cbd5dad2055b64e6a30159742 Reviewed-on: https://chromium-review.googlesource.com/894207 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>