Make it more pleasant to work with expression locations in the

Make it more pleasant to work with expression locations in the
top-level code generator backend.  Introduce function to move one
location to another (source cannot be nowhere); to move registers,
slots, and literals into a location; and to move a location to a
register or slot.

Review URL: http://codereview.chromium.org/338043

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3146 ce2b1a6d-e550-0410-aec6-3dcde31c8c00