After this simply declaring both variables invariant is not treated as
static use.

This simplifies ParseContext a bit, but the effect on compiler
performance tests seems marginal.

BUG=angleproject:2450
TEST=angle_unittests

Change-Id: Ib90cb1d2bd1331542d1cd37732f24efb7833036a
Reviewed-on: https://chromium-review.googlesource.com/992112Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>