Answer from cs61c-av (Kenneth Wong 16192104) for Question 3
The purpose of the separate adder is to compute the branch target as the sum of the incremented PC and the sign-extended shifted left 2 bits.  An ALU cannot be used to replace the function so fthe ALu and adder because we need the adder to decide whether the incremented PC or branch target should replace the PC based on the zero output of the ALU.