[CodeExtractor] Correctly update DT after splitting region header blocks

[mkuper]

Summary

Use a helper from BasicBlockUtils that actually performs the split + update correctly (if DT is not null), instead of doing it part-way like the existing code.

No test, since it's not clear how to test it with any of the in-tree passes that use CE.
BlockExtractor doesn't pass CE a DT, and LoopExtractor requires LoopSimplify, which destroys the problematic pattern (two entries into the region header).

[mkuper]

Forgot llvm-commits. :-\

[davide]

@davidxl fixed the same yesterday.

lib/Transforms/Utils/CodeExtractor.cpp

if (Header != &Header->getParent()->getEntryBlock()) {
     if (NumPredsOutsideRegion <= 1) return;
   }
 
   // Otherwise, we need to split the header block into two pieces: one
   // containing PHI nodes merging values from outside of the region, and a
   // second that contains all of the code for the block and merges back any
   // incoming values from inside of the region.
   BasicBlock::iterator AfterPHIs = Header->getFirstNonPHI();
-  BasicBlock *NewBB = Header->splitBasicBlock(AfterPHIs,
-                                              Header->getName()+".ce");
+  BasicBlock *NewBB = SplitBlock(Header, AfterPHIs, DT);
 
   // We only want to code extract the second block now, and it becomes the new
   // header of the region.
   BasicBlock *OldPred = Header;
   Blocks.remove(OldPred);
   Blocks.insert(NewBB);
   Header = NewBB;
 
-  // Okay, update dominator sets. The blocks that dominate the new one are the
-  // blocks that dominate TIBB plus the new block itself.
-  if (DT)
-    DT->splitBlock(NewBB);
-
   // Okay, now we need to adjust the PHI nodes and any branches from within the
   // region to go to the new header block instead of the old header block.
   if (NumPredsFromRegion) {
     PHINode *PN = cast<PHINode>(OldPred->begin());
     // Loop over all of the predecessors of OldPred that are in the region,
     // changing them to branch to NewBB instead.
     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
       if (Blocks.count(PN->getIncomingBlock(i))) {