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

BUG=angleproject:937
BUG=395048

Change-Id: Ib8d7b98431b8cd3563e1eff8ecc9ed5df1a9b7d6
Reviewed-on: https://chromium-review.googlesource.com/263775Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>