[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 `ml-opt-dev-x86-64` running on `ml-opt-dev-x86-64-b2` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/137/builds/13197

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
56.798 [363/10/3424] Linking CXX static library lib/libLLVMLTO.a
57.909 [363/9/3425] Building AMDGPUGenGlobalISel.inc...
57.911 [363/8/3426] Linking CXX executable bin/lli
58.471 [363/7/3427] Building AMDGPUGenAsmWriter.inc...
58.634 [363/6/3428] Building AMDGPUGenAsmMatcher.inc...
59.744 [363/5/3429] Building AMDGPUGenDAGISel.inc...
61.841 [363/4/3430] Building AMDGPUGenInstrInfo.inc...
65.731 [363/3/3431] Building AMDGPUGenRegisterBank.inc...
66.285 [363/2/3432] Building AMDGPUGenRegisterInfo.inc...
67.999 [362/2/3433] Building CXX object lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o
FAILED: lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/RISCVISelLowering.cpp.o 
ccache /usr/bin/c++ -DCPUINFO_SUPPORTED_PLATFORM=1 -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/ml-opt-dev-x86-64-b1/build/lib/Target/RISCV -I/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV -I/b/ml-opt-dev-x86-64-b1/build/include -I/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/include -isystem /tmp/tflitebuild/tensorflow/include -isystem /tmp/tflitebuild/eigen/include/eigen3 -isystem /tmp/tflitebuild/abseil-cpp/include -isystem /tmp/tflitebuild/flatbuffers/include -isystem /tmp/tflitebuild/gemmlowp/include/gemmlowp -isystem /tmp/tflitebuild/ml_dtypes/src/ml_dtypes -isystem /tmp/tflitebuild/ml_dtypes/src/ml_dtypes/ml_dtypes -isystem /tmp/tflitebuild/ruy/include -isystem /tmp/tflitebuild/cpuinfo/include -isystem /tmp/tflitebuild/ARM_NEON_2_x86_SSE/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -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 -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -fvisibility=hidden  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -DEIGEN_NEON_GEBP_NR=4 -DTFL_STATIC_LIBRARY_BUILD -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/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp: In function ‘llvm::SDValue lowerVECTOR_SHUFFLE(llvm::SDValue, llvm::SelectionDAG&, const llvm::RISCVSubtarget&)’:
/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5778: error: ‘M1VT’ was not declared in this scope; did you mean ‘VT’?
 5778 |     } else if (ContainerVT.bitsGT(M1VT) && isLowSourceShuffle(Mask, VLMAX)) {
      | 
/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5778: error: ‘VLMAX’ was not declared in this scope; did you mean ‘MinVLMAX’?
/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp: At global scope:
/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5358: warning: ‘bool isLowSourceShuffle(llvm::ArrayRef<int>, int)’ defined but not used [-Wunused-function]
 5358 | static bool isLowSourceShuffle(ArrayRef<int> Mask, int Span) {
      | 
84.182 [362/1/3434] Building InstCombineTables.inc...
ninja: build stopped: subcommand failed.

```

</details>

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