public static IntSet makeInterferenceSet(int countRegs) {
    return countRegs <= INTERFERENCE_SET_THRESHOLD_SIZE
        ? new BitIntSet(countRegs)
        : new ListIntSet();
  }
Make IntSet for the interference graph sets. Public because InterferenceGraph is in another
package.

@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