This also makes the instruction constructors private and fixes
all uses of the public constructors to use the factory methods
instead.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23654051

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16808 ce2b1a6d-e550-0410-aec6-3dcde31c8c00