[LoopUnswitch][Dominators] Don't recalculate DominatorTree

[kuhar]

Summary

As of D35528 DominatorTree seems to be preserved correctly and there's no need to recompute it at the end of LoopUnswitch.

I tested this patch by calling DT->verify() instead of DT->recalculate() -- I didn't find any failures when bootstrapping clang and building the llvm test suite.

[kuhar]

Ping

lib/Transforms/Scalar/LoopUnswitch.cpp

bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
 
   bool Changed = false;
   do {
     assert(currentLoop->isLCSSAForm(*DT));
     redoLoop = false;
     Changed |= processCurrentLoop();
   } while(redoLoop);
 
-  // FIXME: Reconstruct dom info, because it is not preserved properly.
-  if (Changed)
-    DT->recalculate(*F);
   return Changed;
 }
 
 // Return true if the BasicBlock BB is unreachable from the loop header.
 // Return false, otherwise.
 bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) {
   auto *Node = DT->getNode(BB)->getIDom();
   BasicBlock *DomBB = Node->getBlock();