[CodeGen] Fix -Wunused-variable in RegisterCoalescer.cpp (NFC)

/llvm-project/llvm/lib/CodeGen/RegisterCoalescer.cpp:1320:18: error: unused variable 'DefSubIdx' [-Werror,-Wunused-variable]
  const unsigned DefSubIdx = DefMI->getOperand(0).getSubReg();
                 ^
1 error generated.

Author: DamonFool

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1317,7 +1317,7 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   if (SrcIdx && DstIdx)
     return false;
 
-  const unsigned DefSubIdx = DefMI->getOperand(0).getSubReg();
+  [[maybe_unused]] const unsigned DefSubIdx = DefMI->getOperand(0).getSubReg();
   const TargetRegisterClass *DefRC = TII->getRegClass(MCID, 0, TRI, *MF);
   if (!DefMI->isImplicitDef()) {
     if (DstReg.isPhysical()) {