The previously-used instruction isn't guaranteed to always be undefined,
and the encoding used was conditional (failing the condition on an
undefined instruction is itself undefined and not guaranteed to
fault!). I would have like to use a more clever encoding (see bug 2963),
but we need the extra bits to encode the size of the constant pool.

BUG=security
R=ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11242002
Patch from JF Bastien <jfb@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12791 ce2b1a6d-e550-0410-aec6-3dcde31c8c00