Refactor code for generating assignments in the fast compiler.

Refactor code for generating assignments in the fast compiler.

1. Refactor the structure of VisitAssignment: The existing code is not ideal
to be extended with support for compound assignments.

2. Reuse common code for keyed property assigments: Now variables rewritten
to a property (.arguments access) are treated like normal keyed property
assignments. This allows us to remove some code duplication.

Review URL: http://codereview.chromium.org/456024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3425 ce2b1a6d-e550-0410-aec6-3dcde31c8c00