[AArch64] Add MATCH loops to LoopIdiomVectorizePass

This patch adds a new loop to LoopIdiomVectorizePass, enabling it to
recognise and use @llvm.experimental.vector.match to vectorise loops
such as:

    char* find_first_of(char *first, char *last,
                        char *s_first, char *s_last) {
      for (; first != last; ++first)
        for (char *it = s_first; it != s_last; ++it)
          if (*first == *it)
            return first;
      return last;
    }

These loops match the C++ standard library's std::find_first_of.

Author: rj-jesus