A Node is a node in the SSA value graph. Every concrete type that implements Node is also either a Value, an Instruction, or both.
Node contains the methods common to Value and Instruction, plus the Operands and Referrers methods generalized to return nil for non-Instructions and non-Values, respectively.
Node is provided to simplify SSA graph algorithms. Clients should use the more specific and informative Value or Instruction interfaces where appropriate.