Refactor l-value tracking to a separate traverser parent class This makes TIntermTraverser implementation easier to understand and removes the overhead of maintaining the user-defined GLSL function table from the traversers that don't need it. Some logic is duplicated between TIntermTraverser and its new subclass TLValueTrackingTraverser, but duplication is hard to eliminate completely since there are some differences scattered throughout the code. BUG=angleproject:1116 TEST=angle_unittests Change-Id: Iab4a0c1d4320ecfafaf18ea3a45824d756890774 Reviewed-on: https://chromium-review.googlesource.com/292721 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>