Clean up checkCanBeLValue Clarify the code so that it will be easier to add marking statically written variables here. BUG=angleproject:2262 TEST=angle_unittests Change-Id: I821bde29beb89e0f3b0f99dc187d2840f8cd0f9b Reviewed-on: https://chromium-review.googlesource.com/977963 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>