[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 `clang-ppc64le-linux-multistage` running on `ppc64le-clang-multistage-test` while building `llvm` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/8162

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

```
Step 4 (build stage 1) failure: 'ninja' (failure)
...
[386/505] Creating library symlink lib/libclangTidyHICPPModule.so
[387/505] Linking CXX shared library lib/libclangTidyPlugin.so.21.0git
[388/505] Creating library symlink lib/libclangTidyPlugin.so
[389/505] Linking CXX shared library lib/libclangTidyMain.so.21.0git
[390/505] Creating library symlink lib/libclangTidyMain.so
[391/505] Linking CXX executable bin/clang-tidy
[392/505] Building CXX object lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/LoopVectorize.cpp.o
[393/505] Linking CXX shared library lib/libclangDaemon.so.21.0git
[394/505] Creating library symlink lib/libclangDaemon.so
[395/505] Linking CXX shared library lib/libLLVMVectorize.so.21.0git
FAILED: lib/libLLVMVectorize.so.21.0git 
: && /usr/lib64/ccache/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-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wl,-z,defs -Wl,-z,nodelete   -Wl,-rpath-link,/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/./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/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/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/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/lib && :
lib/Transforms/Vectorize/CMakeFiles/LLVMVectorize.dir/VPlanTransforms.cpp.o: In function `llvm::VPlanTransforms::simplifyRecipes(llvm::VPlan&, llvm::Type&) [clone .localalias.0]':
VPlanTransforms.cpp:(.text._ZN4llvm15VPlanTransforms15simplifyRecipesERNS_5VPlanERNS_4TypeE+0x11b0): undefined reference to `llvm::VPUnrollPartAccessor<2u>::getUnrollPart(llvm::VPUser&) const'
collect2: error: ld returned 1 exit status
[396/505] Linking CXX shared library lib/libclangdMain.so.21.0git
[397/505] Linking CXX executable bin/clangd-fuzzer
[398/505] Linking CXX executable bin/dexp
[399/505] Linking CXX executable bin/clangd-indexer
[400/505] Linking CXX shared library lib/libclangDaemonTweaks.so.21.0git
ninja: build stopped: subcommand failed.

```

</details>

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