[llvm] [Utils] Consolidate `LockstepReverseIterator` into own header (NFC) (PR #116657)

[Antonio Frighetto via llvm-commits]

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@@ -0,0 +1,160 @@
+//===- LockstepReverseIterator.h ------------------------------*- C++ -*---===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_LOCKSTEPREVERSEITERATOR_H
+#define LLVM_TRANSFORMS_UTILS_LOCKSTEPREVERSEITERATOR_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Instruction.h"
+
+namespace llvm {
+
+struct NoActiveBlocksOption {
+  template <typename... Args> NoActiveBlocksOption(Args...) {}
+};
+
+struct ActiveBlocksOption {
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antoniofrighetto wrote:

I'd rather favour `struct` for consistency, as we keep the default constructor private for `NoActiveBlocksOption`.

https://github.com/llvm/llvm-project/pull/116657