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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/20624

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
15.145 [501/14/6217] Linking CXX static library lib/libMLIRCAPILinalg.a
15.150 [501/13/6218] Linking CXX static library lib/libMLIRGPUToSPIRV.a
15.166 [501/12/6219] Building CXX object tools/mlir/tools/mlir-lsp-server/CMakeFiles/mlir-lsp-server.dir/mlir-lsp-server.cpp.o
15.178 [501/11/6220] Building CXX object tools/mlir/tools/mlir-opt/CMakeFiles/mlir-opt.dir/mlir-opt.cpp.o
15.184 [501/10/6221] Building CXX object tools/mlir/tools/mlir-opt/CMakeFiles/MLIRMlirOptMain.dir/mlir-opt.cpp.o
15.439 [501/9/6222] Building CXX object tools/clang/lib/Driver/CMakeFiles/obj.clangDriver.dir/ToolChains/HIPUtility.cpp.o
15.508 [501/8/6223] Building CXX object tools/mlir/test/lib/Transforms/CMakeFiles/MLIRTestTransforms.dir/TestDialectConversion.cpp.o
15.625 [501/7/6224] Building CXX object tools/mlir/test/lib/Tools/PDLL/CMakeFiles/MLIRTestPDLL.dir/TestPDLL.cpp.o
16.927 [501/6/6225] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
19.697 [501/5/6226] Building CXX object lib/Transforms/Utils/CMakeFiles/LLVMTransformUtils.dir/SimplifyCFG.cpp.o
FAILED: lib/Transforms/Utils/CMakeFiles/LLVMTransformUtils.dir/SimplifyCFG.cpp.o 
ccache /home/buildbots/llvm-external-buildbots/clang.19.1.7/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/lib/Transforms/Utils -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT lib/Transforms/Utils/CMakeFiles/LLVMTransformUtils.dir/SimplifyCFG.cpp.o -MF lib/Transforms/Utils/CMakeFiles/LLVMTransformUtils.dir/SimplifyCFG.cpp.o.d -o lib/Transforms/Utils/CMakeFiles/LLVMTransformUtils.dir/SimplifyCFG.cpp.o -c /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:2503:3: error: reference to 'LockstepReverseIterator' is ambiguous
 2503 |   LockstepReverseIterator<true> LRI(UnconditionalPreds);
      |   ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include/llvm/Transforms/Utils/LockstepReverseIterator.h:41:7: note: candidate found by name lookup is 'llvm::LockstepReverseIterator'
   41 | class LockstepReverseIterator
      |       ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:1787:7: note: candidate found by name lookup is '(anonymous namespace)::LockstepReverseIterator'
 1787 | class LockstepReverseIterator {
      |       ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:2533:44: error: reference to 'LockstepReverseIterator' is ambiguous
 2533 |     auto ProfitableToSinkInstruction = [&](LockstepReverseIterator<true> &LRI) {
      |                                            ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include/llvm/Transforms/Utils/LockstepReverseIterator.h:41:7: note: candidate found by name lookup is 'llvm::LockstepReverseIterator'
   41 | class LockstepReverseIterator
      |       ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp:1787:7: note: candidate found by name lookup is '(anonymous namespace)::LockstepReverseIterator'
 1787 | class LockstepReverseIterator {
      |       ^
2 errors generated.
27.636 [501/4/6227] Building CXX object lib/Transforms/Scalar/CMakeFiles/LLVMScalarOpts.dir/GVNSink.cpp.o
30.844 [501/3/6228] Building CXX object lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/AsmParser.cpp.o
59.827 [501/2/6229] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
61.148 [501/1/6230] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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