Clean up TIntermTraverse API Make NodeInsertMultipleEntry private and clarify some comments in IntermTraverse.h. BUG=angleproject:2100 TEST=angle_unittests, angle_end2end_tests Change-Id: Iae60a46714c8b5cb9ad1e9d70aa6776f9deaf3d5 Reviewed-on: https://chromium-review.googlesource.com/715718 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>