There's a distinction between the two according to the wiki - graveyards are next to places of worship.
so, if you include one in HDM, I'd assume you want to include the other.
As I thought about it, I looked into its usage and found most of the uses of amenity=grave_yard are within the USA: 121,000 of 149,000. (a result of a large GNIS import in the USA in 2008-09 which added features as amenity=grave_yard. Going by the wiki definition and practice, an unknown amount of these imported objects have been changed to landuse=cemetery). As for landuse=cemetery ~16,000 of 139,000 uses are within the USA.
In the end, I'd probably keep both for HDM, although it could be considered whether that distinction is important enough to use 2 different tags.