technically, it belongs to both. however, you have to look at the rails of the cell and how it abuts with its neighbors. the assumption could be that the cell has one VDD rail in M1 while the other VDD is routed like a signal. if the rail-delivered VDD is HIGH, the cell lives in the HIGH domain.
There are different architectures though, so the information you need to find is in the user manual of the cell library you are using.