[XLA] Fix points-to set calculation in HLO ListScheduler.

Previously the list scheduler considered that an instruction used only the buffers defined by its operands. This is inaccurate in the presence of aliasing?an instruction may potentially use anything in the points-to set of the operand, including buffers defined by an ancestor of an operand. Change to use the full points-to set instead.

PiperOrigin-RevId: 189460681