[LoopUnroll] Use addClonedBlockToLoopInfo to add loop header to LI (NFC).

[fhahn]

Summary

I have a similar patch up for review already (D29173). If you prefer I
can squash them both together.

Also I think there more potential for code sharing between
LoopUnroll.cpp and LoopUnrollRuntime.cpp. Do you think patches for
that would be worthwhile?

[mkuper]

Yeah, I think it's the same deal as D29173. The only reservation I have is that this is an optimization, since you avoid a look-up. But that looks like a really minor win to me, and it's better to simplify the code.
So, LGTM, unless @mzolotukhin wants the optimization. :-)

As to general code sharing improvements within the unroller - sounds worthwhile to me.

[fhahn]

Ok great, thanks. I'll hold off committing the change a little longer for @mzolotukhin to raise any objects.

[mzolotukhin]

LGTM!

lib/Transforms/Utils/LoopUnroll.cpp

for (unsigned It = 1; It != Count; ++It) {
     SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
     NewLoops[L] = L;
 
     for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
       ValueToValueMapTy VMap;
       BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
       Header->getParent()->getBasicBlockList().push_back(New);
 
+      assert(*BB != Header || LI->getLoopFor(*BB) == L &&
+             "Header should not be in a sub-loop");
       // Tell LI about New.
-      if (*BB == Header) {
-        assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
-        L->addBasicBlockToLoop(New, *LI);
-      } else {
-        const Loop *OldLoop = addClonedBlockToLoopInfo(*BB, New, LI, NewLoops);
-        if (OldLoop) {
-          LoopsToSimplify.insert(NewLoops[OldLoop]);
 
-          // Forget the old loop, since its inputs may have changed.
-          if (SE)
-            SE->forgetLoop(OldLoop);
-        }
-      }
 
       if (*BB == Header)
         // Loop over all of the PHI nodes in the block, changing them to use
         // the incoming values from the previous block.
         for (PHINode *OrigPHI : OrigPHINode) {
           PHINode *NewPHI = cast<PHINode>(VMap[OrigPHI]);
           Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
           if (Instruction *InValI = dyn_cast<Instruction>(InVal))