ADT: Disallow == and != between pointers and ilist iterators

I completely missed these non-class operators when I removed the
implicit conversions in r252380.  Remove them now.  r261498 should have
already removed all uses.

Note (repeated from r252380): if you have out-of-tree code, it should be
fairly easy to revert this patch downstream while you update your
out-of-tree call sites.  Note that these conversions are occasionally
latent bugs (that may happen to "work" now, but only because of getting
lucky with UB; follow-ups will change your luck).  When they are valid,
I suggest using `->getIterator()` to go from pointer to iterator, and
`&*` to go from iterator to pointer.

llvm-svn: 261499