Revert "[InstCombine] re-commit r218721 with fix for pr21199"

This seems to cause a miscompile when building clang, which causes a
bootstrapped clang to fail or crash in several of its tests.

See:
  http://lab.llvm.org:8013/builders/clang-x86_64-darwin11-RA/builds/1184
  http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/7813

This reverts commit r219282.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219317 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -90,7 +90,6 @@ INITIALIZE_PASS_BEGIN(InstCombiner, "instcombine",
                 "Combine redundant instructions", false, false)
 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
-INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_END(InstCombiner, "instcombine",
                 "Combine redundant instructions", false, false)
 
@@ -98,8 +97,6 @@ void InstCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesCFG();
   AU.addRequired<AssumptionTracker>();
   AU.addRequired<TargetLibraryInfo>();
-  AU.addRequired<DominatorTreeWrapperPass>();
-  AU.addPreserved<DominatorTreeWrapperPass>();
 }
 
 
@@ -2936,9 +2933,12 @@ bool InstCombiner::runOnFunction(Function &F) {
   AT = &getAnalysis<AssumptionTracker>();
   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
   DL = DLP ? &DLP->getDataLayout() : nullptr;
-  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   TLI = &getAnalysis<TargetLibraryInfo>();
 
+  DominatorTreeWrapperPass *DTWP =
+      getAnalysisIfAvailable<DominatorTreeWrapperPass>();
+  DT = DTWP ? &DTWP->getDomTree() : nullptr;
+
   // Minimizing size?
   MinimizeSize = F.getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                                 Attribute::MinSize);