The result can be a register, a stack slot or a constant operand.

Right now it is only used for enviroment uses.

Review URL: http://codereview.chromium.org/6348003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6349 ce2b1a6d-e550-0410-aec6-3dcde31c8c00