Implement gradient operation AST analysis 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/260930 Reviewed-by: Nicolas Capens <capn@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>