Make DT a transitive dependency of LI.

[eugenis]

Summary

LoopInfoWrapperPass::verify uses DT, which means DT must be alive
even if it has no direct users.

Fixes a crash in expensive checks mode.

[eugenis]

This fixes http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/18689.
I've added a pass that preservesCFG; following passes use LI but not DT. Without the transitive dependency, the pass manager kills DT before my pass as there are no future users, but then calls LoopInfoWrapperPass::verify that assumes that DT is still alive.

[pcc]

LGTM

llvm/trunk/lib/Analysis/LoopInfo.cpp

void LoopInfoWrapperPass::verifyAnalysis() const {
   if (VerifyLoopInfo) {
     auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     LI.verify(DT);
   }
 }
 
 void LoopInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
-  AU.addRequired<DominatorTreeWrapperPass>();
+  AU.addRequiredTransitive<DominatorTreeWrapperPass>();
 }
 
 void LoopInfoWrapperPass::print(raw_ostream &OS, const Module *) const {
   LI.print(OS);
 }
 
 PreservedAnalyses LoopVerifierPass::run(Function &F,
                                         FunctionAnalysisManager &AM) {