[turbofan] Add support for ChangeTaggedToUint32 in ChangeLowering.

Also refactor some common code in ChangeLowering.

TEST=cctest,compiler-unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/488043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23216 ce2b1a6d-e550-0410-aec6-3dcde31c8c00