Refactor ChangeLowering class to avoid template specialization.

Also refactor the unit tests and add support to easily
match DAGs using CaptureEq() matcher.

TEST=compiler-unittests
BUG=v8:3489
LOG=n
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/480863002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23140 ce2b1a6d-e550-0410-aec6-3dcde31c8c00