MIPS: Tag smi-constants as smi. This also fixes code that copies holes into arrays.

Port r14814 (8a28ccfb)

BUG=

Review URL: https://codereview.chromium.org/16041005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14830 ce2b1a6d-e550-0410-aec6-3dcde31c8c00