This fixes polymorphic loads to correctly compare in-object offsets
instead of indices, because indices might coincide even though the
actual slot is different because of different instance sizes.

R=danno@chromium.org
BUG=v8:2030
TEST=mjsunit/regress/regress-2030,mjsunit/mirror-array

Review URL: https://chromiumcodereview.appspot.com/9864028

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00