Fix uninitialized variable.

Flags variable was not initialized and later used (both isMBBSafeToOutlineFrom
implementations assume it's initialized), which breaks
test/CodeGen/AArch64/machine-outliner.mir. under memory sanitizer:
MemorySanitizer: use-of-uninitialized-value
    #0  in llvm::AArch64InstrInfo::getOutliningType(llvm::MachineInstrBundleIterator<llvm::MachineInstr, false>&, unsigned int) const llvm/lib/Target/AArch64/AArch64InstrInfo.cpp:5494:9
    #1  in (anonymous namespace)::InstructionMapper::convertToUnsignedVec(llvm::MachineBasicBlock&, llvm::TargetInstrInfo const&) llvm/lib/CodeGen/MachineOutliner.cpp:772:19
    #2  in (anonymous namespace)::MachineOutliner::populateMapper((anonymous namespace)::InstructionMapper&, llvm::Module&, llvm::MachineModuleInfo&) llvm/lib/CodeGen/MachineOutliner.cpp:1543:14
    #3  in (anonymous namespace)::MachineOutliner::runOnModule(llvm::Module&) llvm/lib/CodeGen/MachineOutliner.cpp:1645:3
    #4  in (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&) llvm/lib/IR/LegacyPassManager.cpp:1744:27
    #5  in llvm::legacy::PassManagerImpl::run(llvm::Module&) llvm/lib/IR/LegacyPassManager.cpp:1857:44
    #6  in compileModule(char**, llvm::LLVMContext&) llvm/tools/llc/llc.cpp:597:8

llvm-svn: 346761