Referring a paper which is still under publication
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.