Remove all uses of ilist_node::getNextNode() and ilist_node::getPrevNode() in favor of just using iterators.

This change is needed for the next update to ToT LLVM. It can be put
into place now without breaking anything so I am committing it now.

The churn upstream on ilist_node is neccessary to remove undefined
behavior. Rather than updating the different ilist_node patches for the
hacky change required to not use iterators, just use iterators and keep
everything as ilist_nodes. Upstream they want to eventually do this, so
it makes sense for us to just do it now.

Please do not introduce new invocations of
ilist_node::get{Next,Prev}Node() into the tree.

lib/SILOptimizer/Mandatory/DefiniteInitialization.cpp

@@ -1881,7 +1881,7 @@ SILValue LifetimeChecker::handleConditionalInitAssign() {
   }
   
   // Before the memory allocation, store zero in the control variable.
-  B.setInsertionPoint(TheMemory.MemoryInst->getNextNode());
+  B.setInsertionPoint(&*std::next(TheMemory.MemoryInst->getIterator()));
   SILValue ControlVariableAddr = ControlVariableBox;
   auto Zero = B.createIntegerLiteral(Loc, IVType, 0);
   B.createStore(Loc, Zero, ControlVariableAddr);