TransformUtils: Avoid getNodePtrUnchecked() in integer division, NFC

Stop relying on `getNodePtrUnchecked()` being useful on invalid
iterators.  This function is documented to be for internal use only, and
the pointer type will eventually have to change to remove UB from
ilist_iterator.  Instead, check the iterator before it has been
invalidated.

llvm-svn: 261497