Add a matcher to to check whether two graph are isomorphic.

The goal of this matcher is to indicate whether a rewrite actually does what we
think it does. We check for graph isomorphism, i.e. that two `xnn_subgraph_t`
represent the same graph, even though the in-memory representation may be
different (the `nodes` order for instance).

We take advantage of the fact that our graphs are directed and acyclic to
define a strict order on nodes and sort them.

PiperOrigin-RevId: 908083693

Author: qukhan

test/subgraph/rewrites/BUILD

@@ -0,0 +1,41 @@
+# Copyright 2026 Google LLC
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+load(
+    "//:build_defs.bzl",
+    "xnnpack_cxx_library",
+    "xnnpack_unit_test",
+)
+
+package(default_visibility = ["//:__subpackages__"])
+
+xnnpack_cxx_library(
+    name = "subgraph_matcher",
+    testonly = True,
+    srcs = ["subgraph_matcher.cc"],
+    hdrs = ["subgraph_matcher.h"],
+    deps = [
+        "//:cache",
+        "//:logging",
+        "//:node_type",
+        "//:operator_utils",
+        "//:subgraph",
+        "//:xnnpack_h",
+        "@com_google_googletest//:gtest",  # no g3
+    ],
+)
+
+xnnpack_unit_test(
+    name = "subgraph_matcher_test",
+    srcs = ["subgraph_matcher_test.cc"],
+    deps = [
+        ":subgraph_matcher",
+        "//litert/tensor",
+        "//litert/tensor:arithmetic",
+        "//litert/tensor:datatypes",
+        "//litert/tensor/backends/xnnpack:conversion",
+        "//litert/tensor/utils:matchers_no_g3",
+    ],
+)