static IntSet makeLivenessSet(int countRegs) {
    return countRegs <= LIVENESS_SET_THRESHOLD_SIZE ? new BitIntSet(countRegs) : new ListIntSet();
  }
Make IntSet for register live in/out sets.

@param countRegs {@code >=0;} count of SSA registers used in method
@return {@code non-null;} appropriate set
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SetFactory.java