I'm tempted to suggest "vkil_instruction". "vkil" for "vkd3d-shader intermediate language", of course ;), and "instruction" because I do tend to prefer using proper words where reasonably possible.
That seems fine to me. I'm a little concerned that "vkil" implies Vulkan, which isn't really the case (granted, we already crossed that line with "vkd3d", but we needn't cross it again), but it's probably good enough anyway.