MIPS: Convert String array index/length hash to BitField.

Port r21533 (736c779)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/297233007

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00