MIPS: Symbols for type cells.

Port r19706 (81fd8dd)

Original commit message:
We can make more efficient code to check against type cells in the future if we use symbols, guaranteed not to conflict with user code. Currently, the "symbols" are the hole and undefined. Undefined may come in from the outside.

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/189143006

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19726 ce2b1a6d-e550-0410-aec6-3dcde31c8c00