If depth stencil buffer is disabled during entire renderpass, and at the
end of renderpass we are not storing the data back to memory, then force
the loadOP to DontCare to avoid unnecessary depth stencil load or
clear.

Bug: b/162080462
Change-Id: I30905a6d45bec038de68e7f363ec699eb2be09ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2317726
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>