Fix crash in debug builds introduced with r21110.

Basically we should not check for map deprecation during
code selection, because that may run on the concurrent
compiler thread. So the fix is to move this logic to the
code generation phase instead, which is always run on the
main thread.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/263803005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21112 ce2b1a6d-e550-0410-aec6-3dcde31c8c00