Create new TVariables to change struct declaration qualifiers in
PruneNoOps so that the node doesn't end up with an inconsistent
qualifier from the variable.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I7f6cd557eb3e46f4ead4ffbe37d8960ff7776d10
Reviewed-on: https://chromium-review.googlesource.com/850673Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>