These system values are bound to the same allocation rules as other
semantics: they can share registers with other semantics with the same
interpolation mode and they prefer forming shorter writemasks. However,
for some reason, these don't allow further semantics to share the same
register once allocated, except among themselves.
Similarly to RADV, this is a kind of perverted situation: in
principle Vulkan doesn't allow vkCmdResolveImage() to be
executed conditionally (i.e., it is unaffected by conditional
execution), which means that vkd3d cannot implement conditional
rendering for ResolveSubresource(), hence the todo. However,
like RADV, llvmpipe apparently violates the specification and
still executes the image resolution command conditionally. So
that's a llvmpipe bug, even if one that helps us doing the right
thing.
