Refactor: Make qam16_qam16 more compact

Use local arrays and for-loop to replace duplicated statements.
For reference, here is the table of index v.s. old local variable
suffix.
 0 p1_p1            0 rpi_1_1
 1 p1_p3            1 rpi_1_3
 2 p3_p1            2 rpi_3_1
 3 p3_p3            3 rpi_3_3
 4 p1_m1            4 rmi_1_1
 5 p1_m3            5 rmi_1_3
 6 p3_m1            6 rmi_3_1
 7 p3_m3            7 rmi_3_3
 8 m1_p1
 9 m1_p3
10 m3_p1
11 m3_p3
12 m1_m1
13 m1_m3
14 m3_m1
15 m3_m3