[LCG] Add some basic methods for querying the parent/child relationships
of SCCs in the SCC DAG. Exercise them in the big graph test case. These
will be especially useful for establishing invariants in insertion
logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207749 91177308-0d34-0410-b5e6-96231b3b80d8