This allows us to get rid of totally fake LAST_STRING_TYPE and makes
it possible to test for symbols.

I considered splitting HCheckInstanceType into two instructions, but
it seems nice to be able to hide the instance type implementation
details from the hydrogen level.

Review URL: http://codereview.chromium.org/6964011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7840 ce2b1a6d-e550-0410-aec6-3dcde31c8c00