When createUnaryNode() is called, the child node is guaranteed to be
non-null. This enables simplifying it.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: Ib1d021bbbeab99a6bf1b1be470181e9efbe301c6
Reviewed-on: https://chromium-review.googlesource.com/433467Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>