[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 `polly-x86_64-linux-shared` running on `polly-x86_64-gce2` while building `llvm` at step 5 "build".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/97/builds/5662

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

```
Step 5 (build) failure: 'ninja' (failure)
...
[95/324] Linking CXX shared library lib/libLLVMFrontendOffloading.so.21.0git
[96/324] Linking CXX executable bin/llvm-debuginfod
[97/324] Creating library symlink lib/libLLVMFrontendOffloading.so
[98/324] Linking CXX shared library lib/libLLVMAggressiveInstCombine.so.21.0git
[99/324] Creating library symlink lib/libLLVMAggressiveInstCombine.so
[100/324] Linking CXX shared library lib/libLLVMInstrumentation.so.21.0git
[101/324] Creating library symlink lib/libLLVMInstrumentation.so
[102/324] Linking CXX shared library lib/libLLVMInstCombine.so.21.0git
[103/324] Creating library symlink lib/libLLVMInstCombine.so
[104/324] Linking CXX shared library lib/libLLVMVectorize.so.21.0git
FAILED: lib/libLLVMVectorize.so.21.0git 
: && /usr/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-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 -fuse-ld=lld -Wl,--color-diagnostics   -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/worker/buildbot-workers/polly-x86_64-gce2/rundir/llvm.obj/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/worker/buildbot-workers/polly-x86_64-gce2/rundir/llvm.obj/lib && :
ld.lld: error: undefined symbol: llvm::VPUnrollPartAccessor<2u>::getUnrollPart(llvm::VPUser&) const
>>> referenced by VPlanTransforms.cpp
>>>               lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanTransforms.cpp.o:(llvm::VPlanTransforms::simplifyRecipes(llvm::VPlan&, llvm::Type&) (.localalias))
collect2: error: ld returned 1 exit status
[105/324] Linking CXX shared library lib/libLLVMScalarOpts.so.21.0git
[106/324] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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