verify-uselistorder: Move shuffleUseLists() out of lib/IR

`shuffleUseLists()` is only used in `verify-uselistorder`, so move it
there to avoid bloating other executables.  As a drive-by, update some
of the header docs.

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214592 91177308-0d34-0410-b5e6-96231b3b80d8