Introduce ObjectAccess, which is used by LoadNamedField and StoreNamedField to denote what parts of an object are referred to by a given load or store. Refactor HGraphBuilder to use ObjectAccess, which removes the need to manually set GVN flags and simplifies the code as well.

Review URL: https://codereview.chromium.org/14284010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14791 ce2b1a6d-e550-0410-aec6-3dcde31c8c00