[TableGen] Fix a warning in GlobalISelCombinerEmitter.cpp (NFC)

/llvm-project/llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp:2612:14:
error: variable 'ExpectedID' set but not used [-Werror,-Wunused-but-set-variable]
    unsigned ExpectedID = 0;
             ^
1 error generated.

Author: DamonFool

llvm/utils/TableGen/GlobalISelCombinerEmitter.cpp

@@ -2609,7 +2609,7 @@ void GICombinerEmitter::emitTestSimplePredicate(raw_ostream &OS) {
     // To avoid emitting a switch, we expect that all those rules are in order.
     // That way we can just get the RuleID from the enum by subtracting
     // (GICXXPred_Invalid + 1).
-    unsigned ExpectedID = 0;
+    [[maybe_unused]] unsigned ExpectedID = 0;
     for (const auto &ID : keys(AllCombineRules)) {
       assert(ExpectedID == ID && "combine rules are not ordered!");
       ++ExpectedID;