The method has to be extremely fast as it's very frequently called. It
contributes about 2% cpu time in the DrawCall/gl_null benchmark. With
this optimization it can be decreased to less than 1%.

Bug: chromium:873724

Change-Id: I7fb376db73452dbdf6cb44c92815848e860867c9
Reviewed-on: https://chromium-review.googlesource.com/1179369Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>