void onSourceChanged(SsaInsn insn, RegisterSpec oldSource, RegisterSpec newSource) {
    if (useList == null) return;

    if (oldSource != null) {
      int reg = oldSource.getReg();
      useList[reg].remove(insn);
    }

    int reg = newSource.getReg();
    if (useList.length <= reg) {
      useList = null;
      return;
    }
    useList[reg].add(insn);
  }
Updates the use list for a single change in source register.

@param insn {@code non-null;} insn being changed
@param oldSource {@code null-ok;} The source that was used, if applicable
@param newSource {@code non-null;} the new source being used
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaMethod.java