This will enable sharing code between different AST traversers that
apply transformations on similar node structures. This will make the
code more maintainable.

For now the helper class is used in UnfoldShortCircuitToIf and
SeparateExpressionsReturningArrays.

BUG=angleproject:1341
TEST=angle_end2end_tests, WebGL 2 conformance tests

Change-Id: Ib1e0d5a84fd05bcca983b34f18d47c53e86dc227
Reviewed-on: https://chromium-review.googlesource.com/361693Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>