Because of existing synchronization mechanisms in SwiftShader,
pipeline barriers can be noop for now. If we do remove those
synchronization mechanisms, there's also a simple way of
implementing pipeline barriers, which is commented on here.

Bug b/118619338

Change-Id: If82d275c46f234e5549018115c9f19f21e2c09b2
Reviewed-on: https://swiftshader-review.googlesource.com/c/22708Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>