Rename CountSetBits32 to CountPopulation32 for consistency.

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/502803002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23330 ce2b1a6d-e550-0410-aec6-3dcde31c8c00