Ensure that if-else branches are always sequence nodes This mainly affects RewriteElseBlocks, which was the only piece of code still adding TIntermIfElse nodes directly as children of other TIntermIfElse nodes. BUG=angleproject:1490 TEST=angle_unittests Change-Id: I5b25c2fb9c642424417cd6c29e37c20482c6ffaf Reviewed-on: https://chromium-review.googlesource.com/392847 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>