Improve Array.shift() performance for small arrays.

TEST=mjsunit/array-shift,mjsunit/array-shift2,mjsunit/array-shift3
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/279743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21203 ce2b1a6d-e550-0410-aec6-3dcde31c8c00