Replacing Ideal current source in two-stage opamp
M16 is connected to the M12 to ensure operation in the ohmic region. Recall that ohmic region operation in MOSFET is characterized by the following conditions: 

VDS &lt; VGS - VTH (1)

In M12 the Vds = Vgs = Vdd- Vds(M11)
In M16 the Vds = Vds(M7)-Vgs(M7)

where Vds(M7) = Vdd - Vds(M6)

Therefore for Vds(M16) = Vdd - Vds(M6) - Vgs (M7)

If transistor sizes are identical, the Vds in (M12) should be larger 
than the Vds in (M16) and it ensures ohmic (voltage controlled resistor) region operation. Refer back to (1).

A secondary reason it takes M12 is that M(10) to M(15) are the bias circuit components. You don't want to take biases from the opamp itself. That is the purpose of the bias circuit, to provide reference currents and voltages were needed.