Check the qualifier of a node resulting from the folding of a ternary
node correctly. The folded node might even be a TIntermConstantUnion
with a non-constant qualifier.

BUG=angleproject:2285
TEST=angle_unittests

Change-Id: I74516e44ce9d78bc54093a5b58d14cf33a57e6e5
Reviewed-on: https://chromium-review.googlesource.com/829138Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>