MIPS: Handle holes in smi-untag from LoadKeyed requiring hole handling.

Port r14756 (fda1989f)

BUG=

Review URL: https://codereview.chromium.org/15690003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14761 ce2b1a6d-e550-0410-aec6-3dcde31c8c00