IR: Remove implicit iterator conversions from lib/IR, NFC

Stop converting implicitly between iterators and pointers/references in
lib/IR.  For convenience, I've added a `getIterator()` accessor to
`ilist_node` so that callers don't need to know how to spell the
iterator class (i.e., they can use `X.getIterator()` instead of
`Function::iterator(X)`).

I'll eventually disallow these implicit conversions entirely, but
there's a lot of code, so it doesn't make sense to do it all in one
patch.  One library or so at a time.

Why?  To root out cases of `getNextNode()` and `getPrevNode()` being
used in iterator logic.  The design of `ilist` makes that invalid when
the current node could be at the back of the list, but it happens to
"work" right now because of a bug where those functions never return
`nullptr` if you're using a half-node sentinel.  Before I can fix the
function, I have to remove uses of it that rely on it misbehaving.
(Maybe the function should just be deleted anyway?  But I don't want
deleting it -- potentially a huge project -- to block fixing
ilist/iplist.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249782 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/BasicBlock.cpp

@@ -56,7 +56,7 @@ void BasicBlock::insertInto(Function *NewParent, BasicBlock *InsertBefore) {
   assert(!Parent && "Already has a parent");
 
   if (InsertBefore)
-    NewParent->getBasicBlockList().insert(InsertBefore, this);
+    NewParent->getBasicBlockList().insert(InsertBefore->getIterator(), this);
   else
     NewParent->getBasicBlockList().push_back(this);
 }
@@ -91,26 +91,26 @@ void BasicBlock::setParent(Function *parent) {
 }
 
 void BasicBlock::removeFromParent() {
-  getParent()->getBasicBlockList().remove(this);
+  getParent()->getBasicBlockList().remove(getIterator());
 }
 
 iplist<BasicBlock>::iterator BasicBlock::eraseFromParent() {
-  return getParent()->getBasicBlockList().erase(this);
+  return getParent()->getBasicBlockList().erase(getIterator());
 }
 
 /// Unlink this basic block from its current function and
 /// insert it into the function that MovePos lives in, right before MovePos.
 void BasicBlock::moveBefore(BasicBlock *MovePos) {
-  MovePos->getParent()->getBasicBlockList().splice(MovePos,
-                       getParent()->getBasicBlockList(), this);
+  MovePos->getParent()->getBasicBlockList().splice(
+      MovePos->getIterator(), getParent()->getBasicBlockList(), getIterator());
 }
 
 /// Unlink this basic block from its current function and
 /// insert it into the function that MovePos lives in, right after MovePos.
 void BasicBlock::moveAfter(BasicBlock *MovePos) {
-  Function::iterator I = MovePos;
-  MovePos->getParent()->getBasicBlockList().splice(++I,
-                                       getParent()->getBasicBlockList(), this);
+  MovePos->getParent()->getBasicBlockList().splice(
+      ++MovePos->getIterator(), getParent()->getBasicBlockList(),
+      getIterator());
 }
 
 const Module *BasicBlock::getModule() const {
@@ -196,7 +196,7 @@ BasicBlock::iterator BasicBlock::getFirstInsertionPt() {
   if (!FirstNonPHI)
     return end();
 
-  iterator InsertPt = FirstNonPHI;
+  iterator InsertPt = FirstNonPHI->getIterator();
   if (InsertPt->isEHPad()) ++InsertPt;
   return InsertPt;
 }