MIPS: Fix hole handling, and ensure smi representation is handled properly

Port r14807 (c26f44d6)

BUG=

Review URL: https://codereview.chromium.org/15821007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14812 ce2b1a6d-e550-0410-aec6-3dcde31c8c00