[mlir] Fix a warning

This patch fixes:

  mlir/lib/Transforms/Utils/DialectConversion.cpp:2104:9: error:
  unused variable 'rewriterImpl' [-Werror,-Wunused-variable]

Author: kazutakahirata

Diff for: mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -2106,6 +2106,7 @@ OperationLegalizer::legalizeWithFold(Operation *op,
     rewriterImpl.logger.startLine() << "* Fold {\n";
     rewriterImpl.logger.indent();
   });
+  (void)rewriterImpl;
 
   // Try to fold the operation.
   SmallVector<Value, 2> replacementValues;