@Override
    public void visitBlock(SsaBasicBlock v, SsaBasicBlock parent) {
      DFSInfo bbInfo = new DFSInfo();
      bbInfo.semidom = ++dfsNum;
      bbInfo.rep = v;
      bbInfo.parent = parent;
      vertex.add(v);
      info[v.getIndex()] = bbInfo;
    }
Callback for depth-first walk through control flow graph (either from the entry block or the
exit block). Records the traversal order in the {@code info}list.
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/Dominators.java