[mlir] Fix a warning

This patch fixes:

  mlir/lib/Transforms/Utils/DialectConversion.cpp:1686:14: error:
  unused variable 'newParentOp' [-Werror,-Wunused-variable]

Author: kazutakahirata

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -1700,6 +1700,7 @@ void ConversionPatternRewriterImpl::notifyBlockInserted(
       });
   assert(!wasOpReplaced(newParentOp) &&
          "attempting to insert into a region within a replaced/erased op");
+  (void)newParentOp;
 
   patternInsertedBlocks.insert(block);