gfxbench clears the depth buffer right before the swap. Even though the
last draw call that presents the frame didn't ever read or write to
depth, the tracking we added thought this depth write meant we had to
keep the LOAD_OP as CLEAR. Instead we can refine our check to treat
clears specially when enabling the depth-stencil read-only mode instead
of changing how the tracking works for clears. This way the tracking
can not affect other apps that don't use depth-stencil read-only loops.

Also adds a regression test that counts the clears after a swap.

Bug: angleproject:4959
Bug: angleproject:4979
Change-Id: I12ece6474019f7519a467f827110ad817f7d4df7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2370364
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>