Since we now have a TVariable pointer in TIntermSymbol, we don't need
to look up variables from the symbol table.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Id09610ccec5898eb56dcc5e9112e173edcf7ac97
Reviewed-on: https://chromium-review.googlesource.com/977828Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>