MIPS: Improve Array.shift() performance for small arrays.

Port r21203 (eaa92e4)

TEST=mjsunit/array-shift,mjsunit/array-shift2,mjsunit/array-shift3
BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/275883002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21229 ce2b1a6d-e550-0410-aec6-3dcde31c8c00