Rely on that constant qualification of binary and unary nodes comes
from promote().

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: Ie8d1d4df3c82ae5a2de8cc536e47016d13a4fd3d
Reviewed-on: https://chromium-review.googlesource.com/377960Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>