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/392847Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>