This patch series optimizes programs that use the pattern:

for (;;) {
  glVertexAttribPointer(...)
  glDraw(...)
}

Change 1: Don't null out pipeline on state change.

This allows us to use the transition table for reduced CPU overhead.
Note that we don't use a transition table for compute pipelines.

In total the patch series reduces test iteration time by 25%. This is
the most significant patch, reducing test time by 20%.

Test: DrawCallPerfBenchmark.Run/vulkan_attrib_change
Bug: angleproject:5045
Bug: b/168493024
Change-Id: I3823bb7d83bc2c66357fc5eb8f5dc3e4e54f6a17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2407340
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>