[mlir] Fix -Wunused-but-set-variable in ModuleTranslation.cpp (NFC)

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp:1050:11:
error: variable 'numConstantsHit' set but not used [-Werror,-Wunused-but-set-variable]
      int numConstantsHit = 0;
          ^
mlir/lib/Target/LLVMIR/ModuleTranslation.cpp:1051:11:
error: variable 'numConstantsErased' set but not used [-Werror,-Wunused-but-set-variable]
      int numConstantsErased = 0;
          ^
2 errors generated.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -1047,8 +1047,8 @@ LogicalResult ModuleTranslation::convertGlobals() {
     if (Block *initializer = op.getInitializerBlock()) {
       llvm::IRBuilder<> builder(llvmModule->getContext());
 
-      int numConstantsHit = 0;
-      int numConstantsErased = 0;
+      [[maybe_unused]] int numConstantsHit = 0;
+      [[maybe_unused]] int numConstantsErased = 0;
       DenseMap<llvm::ConstantAggregate *, int> constantAggregateUseMap;
 
       for (auto &op : initializer->without_terminator()) {