CodeGen: MachineInstr::getIterator() => getInstrIterator(), NFC

Delete MachineInstr::getIterator(), since the term "iterator" is
overloaded when talking about MachineInstr.

- Downcast to ilist_node in iplist::getNextNode() and getPrevNode() so
  that ilist_node::getIterator() is still available.
- Add it back as MachineInstr::getInstrIterator().  This matches the
  naming in MachineBasicBlock.
- Add MachineInstr::getBundleIterator().  This is explicitly called
  "bundle" (not matching MachineBasicBlock) to disintinguish it clearly
  from ilist_node::getIterator().
- Update all calls.  Some of these I switched to `auto` to remove
  boiler-plate, since the new name is clear about the type.

There was one call I updated that looked fishy, but it wasn't clear what
the right answer was.  This was in X86FrameLowering::inlineStackProbe(),
added in r252578 in lib/Target/X86/X86FrameLowering.cpp.  I opted to
leave the behaviour unchanged, but I'll reply to the original commit on
the list in a moment.

llvm-svn: 261504

llvm/lib/CodeGen/DFAPacketizer.cpp

@@ -198,7 +198,7 @@ VLIWPacketizerList::~VLIWPacketizerList() {
 void VLIWPacketizerList::endPacket(MachineBasicBlock *MBB, MachineInstr *MI) {
   if (CurrentPacketMIs.size() > 1) {
     MachineInstr *MIFirst = CurrentPacketMIs.front();
-    finalizeBundle(*MBB, MIFirst->getIterator(), MI->getIterator());
+    finalizeBundle(*MBB, MIFirst->getInstrIterator(), MI->getInstrIterator());
   }
   CurrentPacketMIs.clear();
   ResourceTracker->clearResources();