All children of binary and ternary nodes are guaranteed to be
non-null. We don't need to check for their existence when traversing
the tree.

BUG=angleproject:2662
TEST=angle_unittests

Change-Id: I5575058e7213d0c4b4554ca616b4298e535842d6
Reviewed-on: https://chromium-review.googlesource.com/1098670Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>