This refactoring simplifies the compilation code by putting AST checks
and simplification into a separate function. This function will
immediately return false when an error is encountered. This is easier
to maintain than repeated checking of a "success" boolean.

BUG=angleproject:2068
TEST=WebGL conformance tests, angle_unittests, angle_end2end_tests

Change-Id: I1ae1c8def3625ada1482104a6babe605405229ef
Reviewed-on: https://chromium-review.googlesource.com/750085Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>