public IntSet getLiveOutRegs() {
    if (liveOut == null) {
      liveOut = SetFactory.makeLivenessSet(parent.getRegCount());
    }
    return liveOut;
  }
Returns the set of live-out registers. Valid after register interference graph has been
generated, otherwise empty.

@return {@code non-null;} live-out register set
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaBasicBlock.java