Add a helper function to make it easier for traverser classes to insert
statements, and use it in UnfoldShortCircuitToIf.

BUG=angleproject:971
TEST=angle_end2end_tests, WebGL conformance tests

Change-Id: I141bdd8abf4b01988581e6cb27c2320bf38370ac
Reviewed-on: https://chromium-review.googlesource.com/272140Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>