[InstCombine] Re-commit of  r218721 (Optimize icmp-select-icmp sequence)

Fixes the self-host fail. Note that this commit activates dominator
analysis in the combiner by default (like the original commit did).

llvm-svn: 222590
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index e4a4fef..292c206 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -86,6 +86,7 @@
                 "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)
 
@@ -93,6 +94,8 @@
   AU.setPreservesCFG();
   AU.addRequired<AssumptionTracker>();
   AU.addRequired<TargetLibraryInfo>();
+  AU.addRequired<DominatorTreeWrapperPass>();
+  AU.addPreserved<DominatorTreeWrapperPass>();
 }
 
 
@@ -2962,12 +2965,9 @@
   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);