This fixes V8 issue 989.

Before, assignments used the AST ID of the assignment expression to
mark the side effect of the store, which became a target for
deoptimization bailout for code after the assignment.  In effect
contexts this environment included the value of the assignment, which
was unexpected by the unoptimized code.

Now we introduce a new assignment ID for AST node types that include
an assignment (Assignment, CountOperation, and ForInStatement) and use
it for the side effect of the store.

Review URL: http://codereview.chromium.org/5682010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5990 ce2b1a6d-e550-0410-aec6-3dcde31c8c00