ifu: reconsider prediction of prevHalfInstr
     now we do not need to gather meta from the last packet
     because we update with packet, thus updating in the
     correct slot

cputest pass

IFU: fetchPacket should be set invalid when fire and no new data is coming, IBuffer: only flip the valid bit when valid while dequeuing

ibuffer: enqValid should look ahead 16 entries

ibuffer: brInfo is wrapped in brUpdateInfo!