This fixes a minor bug in insertion to a reachable node that caused
DominatorTree.InsertDeleteExhaustive flakiness. The patch also adds
a new testcase for this exact failure.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308074 91177308-0d34-0410-b5e6-96231b3b80d8