[X86] Fix to X86LoadValueInjectionRetHardeningPass for possible segfault

[sconstab]

Summary

MBB.back() could segfault if MBB.empty(). Fixed by checking for MBB.empty() in the loop.

[craig.topper]

LGTM

[sconstab]

@craig.topper Anyone else I should add as a reviewer? Or can we just go ahead and merge?

llvm/lib/Target/X86/X86LoadValueInjectionRetHardening.cpp

LLVM_DEBUG(dbgs() << "Selected register "
                       << Subtarget->getRegisterInfo()->getRegAsmName(ClobberReg)
                       << " to clobber\n");
   } else {
     LLVM_DEBUG(dbgs() << "Could not find a register to clobber\n");
   }
 
   bool Modified = false;
   for (auto &MBB : MF) {
+    if (MBB.empty())
+      continue;
+
     MachineInstr &MI = MBB.back();
     if (MI.getOpcode() != X86::RETQ)
       continue;
 
     if (ClobberReg != X86::NoRegister) {
       MBB.erase_instr(&MI);
       BuildMI(MBB, MBB.end(), DebugLoc(), TII->get(X86::POP64r))
           .addReg(ClobberReg, RegState::Define)