A64: Make simulator compiler with optdebug

For some reason, the compiler cannot deduce the array bounds in opt
debug mode. An explicit check makes it happy.

BUG=none
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/169473003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19408 ce2b1a6d-e550-0410-aec6-3dcde31c8c00