MIPS: Stay in fast enum case if the empty_slow_element_dictionary is used.

Port r18705 (78b4ea3)

Original commit message:
This fixes slow for-in over frozen objects.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/144023002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18721 ce2b1a6d-e550-0410-aec6-3dcde31c8c00