This removes unnecessary indirection. It's easier to just create
TVariables in createSamplerSymbols, and to track referenced variables
using TVariable pointers instead of TIntermSymbol pointers.

BUG=angleproject:2267
TEST=angle_unittests, angle_end2end_tests

Change-Id: Id1e75e04da084eb9026f581f22070b27a45615ba
Reviewed-on: https://chromium-review.googlesource.com/839442Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>