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

BUG=angleproject:937
BUG=395048

Change-Id: Ic31e434c7b5bb6c32eff2336e5949391bfee25a5
Reviewed-on: https://chromium-review.googlesource.com/260930Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>