Also unify Crankshaft code to load array length.

BUG=v8:1493
TEST=external-arrays.js

Review URL: http://codereview.chromium.org/7600025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8901 ce2b1a6d-e550-0410-aec6-3dcde31c8c00