public int oldToNew(int oldReg) {
    if (oldReg >= oldToNew.size()) {
      return -1;
    }

    return oldToNew.get(oldReg);
  }
Returns the new-namespace mapping for the specified old-namespace register, or -1 if one
exists.

@param oldReg {@code >= 0;} old-namespace register
@return new-namespace register or -1 if none
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/BasicRegisterMapper.java