This will allow narrowing down which usages of
[[flatten]] and [[unroll]] are actually useful.

BUG=angleproject:937
BUG=395048

Change-Id: I091e647e3053d22edadd0cabb7c50bd5efa690b2
Reviewed-on: https://chromium-review.googlesource.com/263776Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>