add coherent bit

Author: EmilioLaiso

lib/API/VK/VKResources.h

@@ -26,7 +26,10 @@ inline VkMemoryPropertyFlags getVulkanMemoryFlags(MemoryLocation Location) {
     return VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT |
            VK_MEMORY_PROPERTY_HOST_COHERENT_BIT;
   case MemoryLocation::GpuToCpu:
+    // The specs guarantees that a memory type with both HOST_VISIBLE &
+    // HOST_COHERENT always exists on every conformant device
     return VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT |
+           VK_MEMORY_PROPERTY_HOST_COHERENT_BIT |
            VK_MEMORY_PROPERTY_HOST_CACHED_BIT;
   }
   llvm_unreachable("All MemoryLocation cases handled");