[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.