This commit optimizes the timing of freelist by changing the updating
function of headPtr and tailPtr.

We maintains an one-hot representation of headPtr and further uses it to
read the free registers from the list, which should be better than the
previous implementation where headPtr is used to indexed into the queue.

The update of tailPtr and the freelist is delayed by one cycle to
optimize the timing. Because freelist allocates new registers in the
next cycle iff there are more than RenameWidth free registers in this
cycle. The freed registers in this cycle will never be used in the next
cycle. Thus, we can delay the updating of queue data to the next cycle.
We also move the update of tailPtr to the next cycle, since PopCount
takes a long timing and we move the last adder to the next cycle. Now
the adder works parallely with PopCount. That is, the updating of
tailPtr is pipelined.