AST transformations such as unfolding logical operations can only
create nodes inside sequence (block) nodes. This patch ensures that these
AST transformations work even inside conditional blocks and loop bodies
that were first parsed as single statements instead of blocks.

BUG=angleproject:971
TEST=WebGL conformance tests

Change-Id: If98cb7be653ec7b005537b89547f4f4cf1c07c72
Reviewed-on: https://chromium-review.googlesource.com/272141Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>