This patch series optimizes programs that use the pattern:

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

Change 3: Use FastUnorderedSet for RP XFB buffers.

Most of the time this list is empty. This speeds up the count check for
empty and non-empty sets considerably.

In total the patch series reduces test iteration time by 25%.

Test: DrawCallPerfBenchmark.Run/vulkan_attrib_change
Bug: angleproject:5045
Bug: b/168493024
Change-Id: Ia43eb11bdd62aa3bf425069b591f9318f519fd4c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2409175
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>