JR instruction sets the PC to the content of the register, so we have to
provide a way for this data from the register file (Read data 1 port).
Next, a mux is needed to control whether the PC will take the value coming
from the register file via the added wire or not.
Finally, a signal has to be led from the controller to the newly added mux
to control it.
These three hardware modifications are highlighted in yellow on the
diagram above.
To implement this a new line should be added to the truth table in Figure
5.18 on page 308 for the JR instruction and a new column to produce the
JumpReg signal.
