CodeGen: Turn on sentinel tracking for MachineInstr iterators

This is a prep commit before fixing MachineBasicBlock::reverse_iterator
invalidation semantics, ala r281167 for ilist::reverse_iterator.  This
changes MachineBasicBlock::Instructions to track which node is the
sentinel regardless of LLVM_ENABLE_ABI_BREAKING_CHECKS.

There's almost no functionality change (aside from ABI).  However, in
the rare configuration:

    #if !defined(NDEBUG) && !defined(LLVM_ENABLE_ABI_BREAKING_CHECKS)

the isKnownSentinel() assertions in ilist_iterator<>::operator* suddenly
have teeth for MachineInstr.  If these assertions start firing for your
out-of-tree backend, have a look at the suggestions in the commit
message for r279314, and at some of the commits leading up to it that
avoid dereferencing the end() iterator.

llvm-svn: 281168

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -118,9 +118,9 @@ void ilist_traits<MachineInstr>::removeNodeFromList(MachineInstr *N) {
 
 /// When moving a range of instructions from one MBB list to another, we need to
 /// update the parent pointers and the use/def lists.
-void ilist_traits<MachineInstr>::transferNodesFromList(
-    ilist_traits &FromList, simple_ilist<MachineInstr>::iterator First,
-    simple_ilist<MachineInstr>::iterator Last) {
+void ilist_traits<MachineInstr>::transferNodesFromList(ilist_traits &FromList,
+                                                       instr_iterator First,
+                                                       instr_iterator Last) {
   assert(Parent->getParent() == FromList.Parent->getParent() &&
         "MachineInstr parent mismatch!");
   assert(this != &FromList && "Called without a real transfer...");