Reapply r77654 with a fix: MachineFunctionPass's getAnalysisUsage

shouldn't do AU.setPreservesCFG(), because even though CodeGen passes
don't modify the LLVM IR CFG, they may modify the MachineFunction CFG,
and passes like MachineLoop are registered with isCFGOnly set to true.

llvm-svn: 77691

llvm/lib/CodeGen/LiveVariables.cpp

@@ -48,6 +48,7 @@ static RegisterPass<LiveVariables> X("livevars", "Live Variable Analysis");
 void LiveVariables::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequiredID(UnreachableMachineBlockElimID);
   AU.setPreservesAll();
+  MachineFunctionPass::getAnalysisUsage(AU);
 }
 
 void LiveVariables::VarInfo::dump() const {