Use a root array register for constant loads and stack guards on x64.

Use a root array register for constant loads and stack guards on x64.
This only wins us around 1% in performance, but it makes the code more
compact.  We don't currently have a way to represent in the virtual
frame that a slot contains a value from the root array.  Adding this
would probably make the code more compact.
Review URL: http://codereview.chromium.org/174639

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2783 ce2b1a6d-e550-0410-aec6-3dcde31c8c00