[llvm] [RISCV] Recognize VLA shift pairs from shuffle masks (PR #127710)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/20056

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
18.848 [2508/11/4632] Linking CXX static library lib/libLLVMX86CodeGen.a
18.850 [2507/11/4633] Linking CXX static library lib/libLLVMExegesisAArch64.a
18.853 [2507/10/4634] Linking CXX static library lib/libLLVMExegesisX86.a
21.065 [2507/9/4635] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
21.077 [2506/9/4636] Linking CXX static library lib/libLLVMLTO.a
21.271 [2506/8/4637] Building AMDGPUGenAsmWriter.inc...
22.942 [2506/7/4638] Building AMDGPUGenAsmMatcher.inc...
23.240 [2506/6/4639] Building AMDGPUGenDAGISel.inc...
23.244 [2506/5/4640] Building AMDGPUGenGlobalISel.inc...
23.974 [2506/4/4641] 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 -DLLVM_EXPORTS -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/llvm-x86_64-debian-dylib/build/lib/Target/RISCV -I/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV -I/b/1/llvm-x86_64-debian-dylib/build/include -I/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/include -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/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5723:24: error: reference to local binding 'TrueMask' declared in enclosing function 'lowerVECTOR_SHUFFLE'
        assert(Mask == TrueMask);
                       ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5476:9: note: 'TrueMask' declared here
  auto [TrueMask, VL] = getDefaultVLOps(VT, ContainerVT, DL, DAG, Subtarget);
        ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5728:76: error: reference to local binding 'VL' declared in enclosing function 'lowerVECTOR_SHUFFLE'
                             DAG.getConstant(-SlideAmt, DL, XLenVT), Mask, VL,
                                                                           ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5476:19: note: 'VL' declared here
  auto [TrueMask, VL] = getDefaultVLOps(VT, ContainerVT, DL, DAG, Subtarget);
                  ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5731:71: error: reference to local binding 'VL' declared in enclosing function 'lowerVECTOR_SHUFFLE'
                         DAG.getConstant(SlideAmt, DL, XLenVT), Mask, VL,
                                                                      ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5476:19: note: 'VL' declared here
  auto [TrueMask, VL] = getDefaultVLOps(VT, ContainerVT, DL, DAG, Subtarget);
                  ^
3 errors generated.
24.077 [2506/3/4642] Building AMDGPUGenInstrInfo.inc...
25.727 [2506/2/4643] Building AMDGPUGenRegisterBank.inc...
28.160 [2506/1/4644] Building AMDGPUGenRegisterInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/127710