Optimize VertexArray::bindVertexBufferImpl. Reduces the buffer nullptr checks to a single location. Also optimizes how the transform feedback binding counter is changed. Improves the score of the draw call vertex array change test. Bug: angleproject:3014 Change-Id: I08ff341e08194a407c24143965a3d568e92b97b7 Reviewed-on: https://chromium-review.googlesource.com/c/1406891 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>