registers are required for calls, explicitly construct a Result with
the needed register rather than allocating it.  For returns from
calls, let the return value in r0 be implicit rather than explicitly
allocated.

Review URL: http://codereview.chromium.org/164316

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2659 ce2b1a6d-e550-0410-aec6-3dcde31c8c00