Seeing monomorphic type feedback plus an AccessorPair does not necessarily imply
that the corresponding getter/setter is really there, so we have to check for
this explictly.

TEST=mjsunit/object-define-property

Review URL: https://chromiumcodereview.appspot.com/10825384

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12317 ce2b1a6d-e550-0410-aec6-3dcde31c8c00