Since only for loops can be unrolled, handle each type of loop only
once. This improvement was going to be part of a do-while workaround for
Mac that will instead be done at the AST level.

BUG=angleproject:891

Change-Id: Ic8ecf41aa6d9abd7a01c2230e908c85152f52b62
Reviewed-on: https://chromium-review.googlesource.com/302332
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>