We can combine an initial RenderPass with a read-only RP if the first
RP never writes to depth. We can check the depth-write tracking in
CommandBufferHelper and substitute in a new Framebuffer/RP Desc in this
case as well as issue new layout barriers. We need to disable barrier
merging in this special case.

This reduces the RenderPass count in the Manhattan trace from 15->13.
The performance on the Pixel 4 benchmark goes to ~82% of native for
the on-screen version and ~88% for off-screen. There's also a ~5% bump
in speed for the desktop trace.

Bug: angleproject:4959
Change-Id: I70241824f75eaa1e11b50370f826abc36e91686e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2358772
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>