runtime: minor refactoring in preparation for parallel GC factor sweepspan() out of sweep(), no logical changes
runtime: minor refactoring in preparation for parallel GC
factor sweepspan() out of sweep(), no logical changes

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5991047

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