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

BUG=angleproject:937
BUG=395048

Change-Id: Icc37abf51a1407c9e7bc708dc08d3ba4050a496a
Reviewed-on: https://chromium-review.googlesource.com/261189Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>