Using FAST_SMI_ELEMENTS triggers optimization on 64-bit architectures that load
only the higher 32 bits of the element. If the element is a pointer to undefined
that has 0 in the higher half than it is erroneously treated as SMI 0.

BUG=v8:3158
LOG=N
TEST=mjsunit/sparse-array-reverse,mjsunit/regress/regress-3158.js
R=danno@chromium.org, ishell@chromium.org

Review URL: https://codereview.chromium.org/166653005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19387 ce2b1a6d-e550-0410-aec6-3dcde31c8c00