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

dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go