Callsites now have to manually lookup transitions if required. This
avoids unnecessary overhead of looking up transitions when we don't need
them. This also allows us to use IsFound() in many places where
IsProperty() was required previously.

Review URL: https://chromiumcodereview.appspot.com/10779012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12098 ce2b1a6d-e550-0410-aec6-3dcde31c8c00