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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261497 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed