Vulkan: Add assert to check only clear is staged on swapchain image The swap after clear optimizations on swapchain image multisampled image https://chromium-review.googlesource.com/c/angle/angle/+/6072416 works with the assumption that the only staged updates on swapchain multisampled image is clear. This change adds an assert to validate the assumption holds true before applying the optimization. If the assert is ever triggered, we will know the assumption is violated, and we need to look into how to tweak the optimization or add other optimizations to solve the violated case. Bug: angleproject:382006939 Change-Id: I7ce23dd7767b7730f1fa3ec89cb6616689339602 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6437217 Reviewed-by: mohan maiya <m.maiya@samsung.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Yuxin Hu <yuxinhu@google.com>