[LCG] Make the return of the IntraSCC removal method actually match its
contract (and be much more useful). It now provides exactly the
post-order traversal a caller might need to perform on newly formed
SCCs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207410 91177308-0d34-0410-b5e6-96231b3b80d8