when a node is removed from an ilist, set its next/prev pointers to

when a node is removed from an ilist, set its next/prev pointers to
null.  This means that uses of invalidated iterators will explode violently
with:
ilist:143: failed assertion `NodePtr && "++'d off the end of an ilist!"'

instead of happening to work "most of the time".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48859 91177308-0d34-0410-b5e6-96231b3b80d8