Also has some minor optimizations for the front-end.

12% improvement on the Vulkan VBO change test.

Bug: angleproject:3014
Change-Id: I38e1a8194edfc14bfe57424be348cb9688e928f4
Reviewed-on: https://chromium-review.googlesource.com/c/1369286Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>