The check in addReadDependency to avoid setting self-dependencies was
causing the barrier set in `copySubData` to be potentially set earlier
than the buffer's previous usage. This change allows buffer
self-dependencies to be handled especially.

Bug: angleproject:3194
Change-Id: I08f2c39f420f020ad5faa9735193e6b7142fa756
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1670952
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>