dominated by Root, check that B is available throughout the scope. This
is obviously true (famous last words?) given the current logic, but the
check may be helpful if more complicated reasoning is added one day.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153323 91177308-0d34-0410-b5e6-96231b3b80d8