This functionality is refactored out of EmulatePrecision to be a common
feature of TIntermTraverser. This is done since tracking where l-values
are required will be useful for other traversers. For example, it will
be needed for converting dynamic indexing of matrices and vectors to
function calls. This change adds some overhead to all tree traversers,
but the overhead is expected to be small for typical shaders which don't
contain too many user-defined functions.

BUG=angleproject:1116
TEST=angle_unittests

Change-Id: I54d34c2b5093ef028f2b24d854c11c0195dc1dbb
Reviewed-on: https://chromium-review.googlesource.com/290514Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>