Add a verifier that SemIR is in SSA form. (#7771)

Check that every use of a non-constant instruction is dominated by a
definition of that instruction. Remove the fake (instruction creation
order based) dominance checks in convert; these start spuriously failing
during template instantiation of initializers.

Assisted-by: Claude Opus 5 via Antigravity

toolchain/base/grouped_value_store_test.cpp

@@ -0,0 +1,75 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/base/grouped_value_store.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <utility>
+
+#include "toolchain/base/index_base.h"
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::IsEmpty;
+
+struct TestId : public IndexBase<TestId> {
+  // Only used when a `TestId` is printed, for example by a failing matcher.
+  [[maybe_unused]] static constexpr llvm::StringLiteral Label = "test";
+
+  using IndexBase::IndexBase;
+};
+
+TEST(GroupedValueStore, Empty) {
+  GroupedValueStore<TestId, int> store(0, [](auto /*add*/) {});
+
+  EXPECT_EQ(store.size(), 0U);
+  EXPECT_THAT(store.Get(TestId::None), IsEmpty());
+}
+
+TEST(GroupedValueStore, GroupsValuesById) {
+  std::pair<int, char> values[] = {{2, 'a'}, {0, 'b'}, {2, 'c'}};
+  GroupedValueStore<TestId, char> store(4, [&](auto add) {
+    for (auto [index, value] : values) {
+      add(TestId(index), value);
+    }
+  });
+
+  EXPECT_EQ(store.size(), 3U);
+  EXPECT_THAT(store.Get(TestId(0)), ElementsAre('b'));
+  // A group with no values is empty, not missing.
+  EXPECT_THAT(store.Get(TestId(1)), IsEmpty());
+  // Values within a group are in the order they were enumerated.
+  EXPECT_THAT(store.Get(TestId(2)), ElementsAre('a', 'c'));
+  EXPECT_THAT(store.Get(TestId(3)), IsEmpty());
+}
+
+TEST(GroupedValueStore, IgnoresIdWithNoValue) {
+  GroupedValueStore<TestId, char> store(2, [](auto add) {
+    add(TestId::None, 'a');
+    add(TestId(1), 'b');
+  });
+
+  EXPECT_EQ(store.size(), 1U);
+  EXPECT_THAT(store.Get(TestId::None), IsEmpty());
+  EXPECT_THAT(store.Get(TestId(1)), ElementsAre('b'));
+}
+
+TEST(GroupedValueStore, StoresIdValues) {
+  // An ID type has no default constructor, so this exercises filling the value
+  // array with `None` before overwriting it.
+  GroupedValueStore<TestId, TestId> store(2, [](auto add) {
+    add(TestId(0), TestId(7));
+    add(TestId(0), TestId(8));
+  });
+
+  EXPECT_THAT(store.Get(TestId(0)), ElementsAre(TestId(7), TestId(8)));
+  EXPECT_THAT(store.Get(TestId(1)), IsEmpty());
+}
+
+}  // namespace
+}  // namespace Carbon::Testing