private void addEdge(EscapeSet parentSet, EscapeSet childSet) {
    if (!childSet.parentSets.contains(parentSet)) {
      childSet.parentSets.add(parentSet);
    }
    if (!parentSet.childSets.contains(childSet)) {
      parentSet.childSets.add(childSet);
    }
  }
Creates a link in the lattice between two EscapeSets due to a put instruction. The object being
put is the child and the object being put into is the parent. A child set must always have an
escape state at least as high as its parent.

@param parentSet {@code non-null;} the EscapeSet for the object being put into
@param childSet {@code non-null;} the EscapeSet for the object being put
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/EscapeAnalysis.java