To avoid deopts a few extra changes were needed:

o Enable megamorphic state for special property loads on
  primitives. We used to flip between monomorphic stubs.
o Extract pure string (no string wrapper support) version of the
  string length stub.

Review URL: http://codereview.chromium.org/6334015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00