Since a6242e4d ("Vulkan: Support submitting multiple semaphores"),
semaphores are not generated because the function generating checks for
non-empty command graph right after flushing.

Bug: angleproject:3492

Change-Id: I03db4a77fe24fdfdf39ba535b235451ae0275a0e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1638579Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>