public BitSet compatibleRegs(DalvInsn insn) {
    return new BitSet();
  }
Returns which of a given instruction's registers will fit in this instance's format.

<p>The default implementation of this method always returns an empty BitSet. Subclasses must
override this method if they have registers.

@param insn {@code non-null;} the instruction to check
@return {@code non-null;} a BitSet flagging registers in the register list that are compatible
    to this format
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/InsnFormat.java