The previous code was hard to read since the referenced interface
blocks stored a different type of node depending on if the interface
block was instanced or not.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ie8fdb61a17280ca0875159702f819b884d08706b
Reviewed-on: https://chromium-review.googlesource.com/839443
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>