math/big: optimize common case of Int.Bit(0)
math/big: optimize common case of Int.Bit(0)

R=golang-dev, dsymonds, rsc
CC=golang-dev
http://codereview.appspot.com/6306069

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang