MIPS: Replace tagged keys for fast access by smi, and use smi in boundscheck.

Port r14794 (05b08564)

BUG=

Review URL: https://codereview.chromium.org/15937009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14810 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: DummyUse if Representation::Tagged()+Type::Smi() to Representation::Smi()

Port r14802 (d4a53e02)

BUG=

Review URL: https://codereview.chromium.org/16019009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14811 ce2b1a6d-e550-0410-aec6-3dcde31c8c00