Fix to get around an assertion that triggers when generating code that happens to be dead because the assertion is checked a bit earlier at runtime.

R=ishell@chromium.org
BUG=355486
LOG=N

Review URL: https://codereview.chromium.org/201573011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20218 ce2b1a6d-e550-0410-aec6-3dcde31c8c00