This is a small optimization for the WebGL compatibility mode.
Instead of scanning the list of attributes for a Transform feedback
conflict, it can quickly check a cached mask. This should save a lot
of cycles on the fast path.

Bug: angleproject:1391
Change-Id: Icb8d095493a629dbff0e93872357e5bf7c7458ae
Reviewed-on: https://chromium-review.googlesource.com/1011236
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>