[RegScavenger][NFC] Refer to the already initialized local variable for spill slot index

llvm/lib/CodeGen/RegisterScavenging.cpp

if (!TRI->saveScavengerRegister(*MBB, Before, UseMI, &RC, Reg)) {
     // Spill the scavenged register before \p Before.
     int FI = Scavenged[SI].FrameIndex;
     if (FI < FIB || FI >= FIE) {
       std::string Msg = std::string("Error while trying to spill ") +
           TRI->getName(Reg) + " from class " + TRI->getRegClassName(&RC) +
           ": Cannot scavenge register without an emergency spill slot!";
       report_fatal_error(Msg.c_str());
     }
-    TII->storeRegToStackSlot(*MBB, Before, Reg, true, Scavenged[SI].FrameIndex,
-                             &RC, TRI);
+    TII->storeRegToStackSlot(*MBB, Before, Reg, true, FI, &RC, TRI);
     MachineBasicBlock::iterator II = std::prev(Before);
 
     unsigned FIOperandNum = getFrameIndexOperandNum(*II);
     TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
 
     // Restore the scavenged register before its use (or first terminator).
-    TII->loadRegFromStackSlot(*MBB, UseMI, Reg, Scavenged[SI].FrameIndex,
-                              &RC, TRI);
+    TII->loadRegFromStackSlot(*MBB, UseMI, Reg, FI, &RC, TRI);
     II = std::prev(UseMI);
 
     FIOperandNum = getFrameIndexOperandNum(*II);
     TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
   }
   return Scavenged[SI];
 }