Referring a paper which is still under publication My domain is Image Processing and Computer Vision.

I've written a follow-up paper for my previous paper which is still not available in IEEE Xplore. I am submitting the new paper in a conference. I want to ask what is the best approach for making the previous paper available to reviewers?


Upload the previous paper in arXiv
Submit the previous paper as a supplementary material 


Please guide me.
I would make a distinction between whether your paper has been presented at a conference and whether it has appeared in IEEE Xplore.  

If it has already been presented, then it's out there in the community and, in my opinion, it is entirely reasonable to take advantage of IEEE's self-archiving options and post it on a personal or institutional website where it can be readily found by search engines.  There is no reason to wait for the (sometimes painfully slow) updating of the IEEE Xplore database.

If it has yet to appear, however, then I would recommend instead attaching it as supplementary material, if possible.