[mlir] Fix the build in the release mode

This patch fixes:

  mlir/lib/Dialect/Linalg/Transforms/Transforms.cpp:286:47: error: use
  of undeclared identifier 'stringifyReassocIndices'

  mlir/lib/Dialect/Linalg/Transforms/Transforms.cpp:412:47: error: use
  of undeclared identifier 'stringifyReassocIndices'

Author: kazutakahirata

mlir/lib/Dialect/Linalg/Transforms/Transforms.cpp

@@ -89,11 +89,11 @@ static bool hasAtMostOneResultFunctionOfDim(AffineMap map, int64_t dim) {
   }
   return true;
 }
+#endif // NDEBUG
 
 static std::string stringifyReassocIndices(ReassociationIndicesRef ri) {
   return llvm::interleaved(ri, ", ", /*Prefix=*/"|", /*Suffix=*/"");
 }
-#endif // NDEBUG
 
 /// Return the index of the first result of `map` that is a function of
 /// AffineDimExpr(dim), std::nullopt otherwise.