The instanceoff stub now supports arguments in registers and checks for null,
smi and string values without going to runtime.

Needs porting to X64 and ARM.
Review URL: http://codereview.chromium.org/5828004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6042 ce2b1a6d-e550-0410-aec6-3dcde31c8c00