This works around a Mac driver shader compiler bug that makes many
do-while loops cause GPU-hangs when ran.

BUG=angleproject:891

Change-Id: I29828d6ea9e887ad0ed0c577f1deb41fb632a900
Reviewed-on: https://chromium-review.googlesource.com/302465Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>