[llvm] [RISCV] Allow undef prefix for local repeating VLA shuffle lowering (PR #126097)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-debian-fast` running on `gribozavr4` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/18443

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
2 warnings generated.
34.894 [556/7/5582] Linking CXX static library lib/libLLVMLTO.a
34.975 [555/7/5583] Linking CXX static library lib/libclangCodeGen.a
34.980 [553/8/5584] Linking CXX static library lib/libclangFrontendTool.a
34.988 [552/8/5585] Linking CXX static library lib/libclangInterpreter.a
35.117 [552/7/5586] Building AMDGPUGenDAGISel.inc...
35.193 [552/6/5587] Building AMDGPUGenGlobalISel.inc...
36.258 [552/5/5588] Building AMDGPUGenInstrInfo.inc...
37.057 [552/4/5589] Linking CXX executable bin/clang-import-test
37.637 [552/3/5590] Building CXX object lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o
FAILED: lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /usr/bin/ccache /usr/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/b/1/clang-x86_64-debian-fast/llvm.obj/lib/Target/RISCV -I/b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV -I/b/1/clang-x86_64-debian-fast/llvm.obj/include -I/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include -std=c++11 -Wdocumentation -Wno-documentation-deprecated-sync -fPIC -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 -fvisibility=hidden  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -std=c++17 -MD -MT lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o -MF lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o.d -o lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o -c /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:14:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.h:17:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCV.h:18:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Target/TargetMachine.h:19:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:30:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/TargetParser/ARMTargetParser.h:267:12: warning: parameter 'Arch' not found in the function declaration [-Wdocumentation]
/// \param Arch the architecture name (e.g., "armv7s"). If it is an empty
           ^~~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/TargetParser/ARMTargetParser.h:267:12: note: did you mean 'MArch'?
/// \param Arch the architecture name (e.g., "armv7s"). If it is an empty
           ^~~~
           MArch
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:14:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.h:17:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCV.h:18:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/Target/TargetMachine.h:19:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:630:14: warning: parameter 'Sym' not found in the function declaration [-Wdocumentation]
  /// \param Sym - The symbol on the .ref directive.
             ^~~
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/MC/MCStreamer.h:630:14: note: did you mean 'Symbol'?
  /// \param Sym - The symbol on the .ref directive.
             ^~~
             Symbol
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:14:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.h:20:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/SelectionDAG.h:28:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/MachineFunction.h:26:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/MachineBasicBlock.h:21:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/MachineInstr.h:25:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/MachineOperand.h:307:14: warning: parameter 'IntrinsicInfo' not found in the function declaration [-Wdocumentation]
  /// \param IntrinsicInfo - same as \p TRI.
             ^~~~~~~~~~~~~
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:14:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Target/RISCV/RISCVISelLowering.h:20:
In file included from /b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/SelectionDAG.h:31:
/b/1/clang-x86_64-debian-fast/llvm.src/llvm/include/llvm/CodeGen/SelectionDAGNodes.h:2223:10: warning: HTML start tag prematurely ended, expected attribute name or '>' [-Wdocumentation]
  /// "<a, a+n, a+2n, a+3n, ...>" where a is integer and n is a non-zero integer,

```

</details>

https://github.com/llvm/llvm-project/pull/126097