we assumed a CFG structure that would be valid when all code in
the function is reachable, but not all code is necessarily
reachable. Do a simple, but horrible, CFG walk to check for this
case.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62487 91177308-0d34-0410-b5e6-96231b3b80d8
