This is just refactoring to clean up some duplicate code. A new test
is added to make sure this doesn't break UnfoldShortCircuitAST.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I108e646f71ba631cbd5dad2055b64e6a30159742
Reviewed-on: https://chromium-review.googlesource.com/894207Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>