def to_risc(compiler)
      l_reg = left.to_register(compiler, self)
      r_reg = right.to_register(compiler, self)
      compiler.add_code Risc.op( self , :- , l_reg , r_reg)
      compiler.add_code Risc::IsZero.new( self, false_jump.risc_label(compiler))
    end
def to_risc(compiler)
      l_reg = left.to_register(compiler, self)
      r_reg = right.to_register(compiler, self)
      compiler.add_code Risc.op( self , :- , l_reg , r_reg)
      compiler.add_code Risc::IsZero.new( self, false_jump.risc_label(compiler))
    end
basically move both left and right values into register
 subtract them and see if IsZero comparison