Implemented waitEvents as a sync operation, like pipeline barriers,
since, as the Vulkan spec states:
"vkCmdWaitEvents is used with vkCmdSetEvent to define a memory
 dependency between two sets of action commands, roughly in the
 same way as pipeline barriers, but split into two commands such
 that work between the two may execute unhindered."

Only the pEvents parameter is supported and currently doesn't support
pMemoryBarrier, pBufferMemoryBarriers or pImageMemoryBarriers.

Bug b/118620868

Change-Id: I30ccc65c65dfc7d9a99e25ebb535061c618375cb
Tests: dEQP-VK.api.command_buffers.record_simul_use_primary
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27348Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>