ADT: Do not inherit from std::iterator in ilist_iterator

Inheriting from std::iterator uses more boiler-plate than manual
typedefs.  Avoid that in both ilist_iterator and
MachineInstrBundleIterator.

This has the side effect of removing ilist_iterator from certain ADL
lookups in namespace std; calls to std::next need to be qualified by
"std::" that didn't have to before.  The one case of this in-tree was
operating on a temporary, so I used the more compact operator++.

llvm-svn: 280570

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -2226,7 +2226,7 @@ void BoUpSLP::setInsertPointAfterBundle(ArrayRef<Value *> VL) {
 
   // Set the insertion point after the last instruction in the bundle. Set the
   // debug location to Front.
-  Builder.SetInsertPoint(BB, next(BasicBlock::iterator(LastInst)));
+  Builder.SetInsertPoint(BB, ++LastInst->getIterator());
   Builder.SetCurrentDebugLocation(Front->getDebugLoc());
 }