[mlir] Fix a warning

This patch fixes:

  mlir/lib/Transforms/Utils/DialectConversion.cpp:2761:9: error:
  unused variable 'impl' [-Werror,-Wunused-variable]

Author: kazutakahirata

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -2758,7 +2758,7 @@ LogicalResult OperationLegalizer::legalizePatternResult(
     const SetVector<Operation *> &newOps,
     const SetVector<Operation *> &modifiedOps,
     const SetVector<Block *> &insertedBlocks) {
-  auto &impl = rewriter.getImpl();
+  [[maybe_unused]] auto &impl = rewriter.getImpl();
   assert(impl.pendingRootUpdates.empty() && "dangling root updates");
 
 #if MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS