MachineVerifier: avoid reference to nullptr

[svenvh]

Summary

Instantiation of the MachineVerifierPass through
PassInfo::getNormalCtor would yield a segfault since the default
constructor of the MachineVerifierPass takes a reference to nullptr.

Patch by Simone Pellegrini

[dblaikie]

Looks good (optionally, if you like, you could switch the parameter type to std::string value (not reference) and initialize the member with std::move of the parameter)

CodeGen/MachineVerifier.cpp

struct MachineVerifier {
     void verifySlotIndexes() const;
     void verifyProperties(const MachineFunction &MF);
   };
 
   struct MachineVerifierPass : public MachineFunctionPass {
     static char ID; // Pass ID, replacement for typeid
     const std::string Banner;
 
-    MachineVerifierPass(const std::string &banner = nullptr)
+    MachineVerifierPass(const std::string &banner = std::string())
       : MachineFunctionPass(ID), Banner(banner) {
         initializeMachineVerifierPassPass(*PassRegistry::getPassRegistry());
       }
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.setPreservesAll();
       MachineFunctionPass::getAnalysisUsage(AU);
     }