Move SamplerTexturePair from inspector to sem.

The plan is to reuse this datatype for the combine-samplers transform.

Bug: tint:1366
Change-Id: Icd2f4bd45b662f32fe9803e3485f1a54a2c42265
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/76320
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/sem/sampler_texture_pair.h

@@ -0,0 +1,71 @@
+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_SEM_SAMPLER_TEXTURE_PAIR_H_
+#define SRC_SEM_SAMPLER_TEXTURE_PAIR_H_
+
+#include <cstdint>
+#include <functional>
+
+#include "src/sem/binding_point.h"
+
+namespace tint {
+namespace sem {
+
+/// Mapping of a sampler to a texture it samples.
+struct SamplerTexturePair {
+  /// group & binding values for a sampler.
+  BindingPoint sampler_binding_point;
+  /// group & binding values for a texture samepled by the sampler.
+  BindingPoint texture_binding_point;
+
+  /// Equality operator
+  /// @param rhs the SamplerTexturePair to compare against
+  /// @returns true if this SamplerTexturePair is equal to `rhs`
+  inline bool operator==(const SamplerTexturePair& rhs) const {
+    return sampler_binding_point == rhs.sampler_binding_point &&
+           texture_binding_point == rhs.texture_binding_point;
+  }
+
+  /// Inequality operator
+  /// @param rhs the SamplerTexturePair to compare against
+  /// @returns true if this SamplerTexturePair is not equal to `rhs`
+  inline bool operator!=(const SamplerTexturePair& rhs) const {
+    return !(*this == rhs);
+  }
+};
+
+}  // namespace sem
+}  // namespace tint
+
+namespace std {
+
+/// Custom std::hash specialization for tint::sem::SamplerTexturePair so
+/// SamplerTexturePairs be used as keys for std::unordered_map and
+/// std::unordered_set.
+template <>
+class hash<tint::sem::SamplerTexturePair> {
+ public:
+  /// @param stp the texture pair to create a hash for
+  /// @return the hash value
+  inline std::size_t operator()(
+      const tint::sem::SamplerTexturePair& stp) const {
+    return tint::utils::Hash(stp.sampler_binding_point,
+                             stp.texture_binding_point);
+  }
+};
+
+}  // namespace std
+
+#endif  // SRC_SEM_SAMPLER_TEXTURE_PAIR_H_