Until now we only supported postfix operations on global variables.
This change add generic count operations to the top-level compiler.

I tried to re-use code from the code generator used for assignment expressions
where possible.

Review URL: http://codereview.chromium.org/496009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3530 ce2b1a6d-e550-0410-aec6-3dcde31c8c00