[X86] Fix -Wunused-variable in X86InstrInfo.cpp (NFC)

llvm-project/llvm/lib/Target/X86/X86InstrInfo.cpp:3467:14:
error: unused variable 'isMemOp' [-Werror,-Wunused-variable]
 3467 |   const auto isMemOp = [](const MCOperandInfo &OpInfo) -> bool {
      |              ^~~~~~~
1 error generated.

Author: DamonFool

llvm/lib/Target/X86/X86InstrInfo.cpp

@@ -3464,9 +3464,11 @@ bool X86::isX87Instruction(MachineInstr &MI) {
 }
 
 int X86::getFirstAddrOperandIdx(const MachineInstr &MI) {
+#ifdef EXPENSIVE_CHECKS
   const auto isMemOp = [](const MCOperandInfo &OpInfo) -> bool {
     return OpInfo.OperandType == MCOI::OPERAND_MEMORY;
   };
+#endif
 
   const MCInstrDesc &Desc = MI.getDesc();