[TableGen] Silence a warning (NFC)

/llvm-project/llvm/utils/TableGen/Common/CodeGenTarget.cpp:286:12:
 error: variable 'SkippedInsts' set but not used [-Werror,-Wunused-but-set-variable]
  unsigned SkippedInsts = 0;
           ^
1 error generated.

Author: DamonFool

llvm/utils/TableGen/Common/CodeGenTarget.cpp

@@ -283,7 +283,7 @@ void CodeGenTarget::ComputeInstrsByEnum() const {
   assert(EndOfPredefines == getNumFixedInstructions() &&
          "Missing generic opcode");
 
-  unsigned SkippedInsts = 0;
+  [[maybe_unused]] unsigned SkippedInsts = 0;
 
   for (const auto &[_, CGIUp] : InstMap) {
     const CodeGenInstruction *CGI = CGIUp.get();