Add option to disallow modref function calls in scops.

Add option to disallow modref function calls in scops.

This might be useful to evaluate the benefit of us handling modref funciton
calls. Also, a new bug that was triggered by modref function calls was
recently reported http://llvm.org/PR27035. To ensure the same issue does not
cause troubles for other people, we temporarily disable this until the bug
is resolved.

llvm-svn: 264140