[llvm] [VPlan] Remove no-op SCALAR-STEPS after unrolling. (PR #123655)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-latest-gcc` running on `linaro-flang-aarch64-latest-gcc` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/130/builds/11595

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
198.467 [3801/12/3624] Copying clang's prfchwintrin.h...
198.467 [3801/11/3625] Copying clang's ptrauth.h...
198.468 [3801/10/3626] Copying clang's ptwriteintrin.h...
198.468 [3801/9/3627] Building LinalgRelayoutOps.cpp.inc...
198.469 [3801/8/3628] Copying clang's raointintrin.h...
198.469 [3801/7/3629] Copying clang's rdpruintrin.h...
198.470 [3801/6/3630] Copying clang's rdseedintrin.h...
198.470 [3801/5/3631] Copying clang's rtmintrin.h...
198.483 [3801/4/3632] Building LinalgRelayoutOps.h.inc...
198.727 [3801/3/3633] Linking CXX shared library lib/libLLVMVectorize.so.21.0git
FAILED: lib/libLLVMVectorize.so.21.0git 
: && /usr/local/bin/c++ -fPIC -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wl,-z,defs -Wl,-z,nodelete   -Wl,-rpath-link,/home/tcwg-buildbot/worker/flang-aarch64-latest-gcc/build/./lib  -Wl,--gc-sections -shared -Wl,-soname,libLLVMVectorize.so.21.0git -o lib/libLLVMVectorize.so.21.0git lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/LoadStoreVectorizer.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/LoopIdiomVectorize.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/LoopVectorizationLegality.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/LoopVectorize.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/DependencyGraph.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/InstrMaps.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Interval.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Legality.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/BottomUpVec.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/RegionsFromBBs.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/RegionsFromMetadata.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/SeedCollection.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/TransactionAcceptOrRevert.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Passes/TransactionSave.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/SandboxVectorizer.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/SandboxVectorizerPassBuilder.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/Scheduler.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/SeedCollector.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SandboxVectorizer/VecUtils.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/SLPVectorizer.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/Vectorize.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VectorCombine.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlan.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanAnalysis.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanConstruction.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanHCFGBuilder.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanRecipes.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanSLP.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanTransforms.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanUnroll.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanVerifier.cpp.o lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanUtils.cpp.o  -Wl,-rpath,"\$ORIGIN/../lib:/home/tcwg-buildbot/worker/flang-aarch64-latest-gcc/build/lib:"  lib/libLLVMTransformUtils.so.21.0git  lib/libLLVMSandboxIR.so.21.0git  lib/libLLVMAnalysis.so.21.0git  lib/libLLVMCore.so.21.0git  lib/libLLVMSupport.so.21.0git  -Wl,-rpath-link,/home/tcwg-buildbot/worker/flang-aarch64-latest-gcc/build/lib && :
/usr/bin/ld: lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanTransforms.cpp.o: in function `llvm::VPlanTransforms::simplifyRecipes(llvm::VPlan&, llvm::Type&) [clone .localalias]':
VPlanTransforms.cpp:(.text._ZN4llvm15VPlanTransforms15simplifyRecipesERNS_5VPlanERNS_4TypeE+0xe80): undefined reference to `llvm::VPUnrollPartAccessor<2u>::getUnrollPart(llvm::VPUser&) const'
collect2: error: ld returned 1 exit status
198.957 [3801/2/3634] Building AArch64GenSubtargetInfo.inc...
200.183 [3801/1/3635] Building AArch64GenInstrInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/123655