void onSourcesChanged(SsaInsn insn, RegisterSpecList oldSources) {
    if (useList == null) return;

    if (oldSources != null) {
      removeFromUseList(insn, oldSources);
    }

    RegisterSpecList sources = insn.getSources();
    int szNew = sources.size();

    for (int i = 0; i < szNew; i++) {
      int reg = sources.get(i).getReg();
      useList[reg].add(insn);
    }
  }
Updates the use list for a source list change.

@param insn {@code insn non-null;} insn being changed. {@code insn.getSources()} must return
    the new source list.
@param oldSources {@code null-ok;} list of sources that were previously used
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaMethod.java