When block nodes were being created for loop bodies that didn't have
braces in the parsed source, the code didn't check if the loop body
was a declaration node without children. Always use appendStatement()
for adding statements to a block, so that declaration nodes without
children don't end up in the AST.

Similarly make sure that loop init nodes aren't declarations without
children.

BUG=chromium:712550
TEST=angle_end2end_tests

Change-Id: I5e79b700fe6158fa2422fcf4cd13818b2bd24863
Reviewed-on: https://chromium-review.googlesource.com/481660Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>