I would like to merge G1UpdateRegionLivenessAndSelectForRebuildTask and G1BuildCandidateRegionsTask from G1CollectionSetChooser since the latter mostly repeats the work of the former (now).

As a first step, move G1UpdateRegionLivenessAndSelectForRebuildTask into its own file to later merge them there to reduce the noise in the g1ConcurrentMark file.