Relying on the AST conforming to known limitations makes the code for
recording referenced varyings easier to understand.

BUG=angleproject:2104
TEST=angle_end2end_tests

Change-Id: Icdcd7602f6ed54fa439f989bf256e261627d11f5
Reviewed-on: https://chromium-review.googlesource.com/568018Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>