Port r9812 (c1644a9d)

Note: MIPS already had the fix for this regression.
This commit only adds the extra assertions.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8343030
Patch from Gergely Kis <gergely@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9827 ce2b1a6d-e550-0410-aec6-3dcde31c8c00