Spiff up the comment on this method, making the example a bit more
pretty in doxygen, adding some of the details actually present in
a classic example where this matters (a loop from gzip and many other
compression algorithms), and a cautionary note about the risks inherent
in the transform. This has come up on the mailing lists recently, and
I suspect folks reading this code could benefit from going and looking
at the MI pass that can really deal with these issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173329 91177308-0d34-0410-b5e6-96231b3b80d8