[llvm] [ADT] Add and use (for AArch64) `ValueWithSentinel<T, Sentinel>` (PR #158120)

[Jakub Kuderski via llvm-commits]

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@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines the ValueWithSentinel class, which is a type akin to a
+/// std::optional, but uses a sentinel rather than an additional "valid" flag.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_VALUEWITHSENTINEL_H
+#define LLVM_ADT_VALUEWITHSENTINEL_H
+
+#include <cassert>
+#include <limits>
+#include <utility>
+
+namespace llvm {
+
+template <typename T, T Sentinel> class ValueWithSentinel {
+public:
+  ValueWithSentinel() = default;
+
+  ValueWithSentinel(T Value) : Value(std::move(Value)) {
+    assert(Value != Sentinel && "Value is sentinel (use default constructor)");
+  };
+
+  ValueWithSentinel &operator=(T const &NewValue) {
+    assert(NewValue != Sentinel && "Assigned to sentinel (use .clear())");
+    Value = NewValue;
+    return *this;
+  }
+
+  bool operator==(ValueWithSentinel const &Other) const {
+    return Value == Other.Value;
+  }
+
+  bool operator!=(ValueWithSentinel const &Other) const {
+    return !(*this == Other);
+  }
+
+  T &operator*() {
+    assert(has_value() && "Invalid value");
+    return Value;
+  }
+  const T &operator*() const {
+    return const_cast<ValueWithSentinel &>(*this).operator*();
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kuhar wrote:

Could we do `return Value;` here as well?

https://github.com/llvm/llvm-project/pull/158120