[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-clang-x86_64-expensive-checks-debian` running on `gribozavr4` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/16/builds/14101

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
20.972 [494/11/3594] Building OCaml stub object file executionengine_ocaml.o
20.987 [493/11/3595] Building OCaml library llvm_X86
21.020 [492/11/3596] Building OCaml library llvm_executionengine
21.028 [491/11/3597] Building OCaml documentation for llvm_X86
21.037 [490/11/3598] Running utility command for ocaml_llvm_X86
21.119 [490/10/3599] Building OCaml documentation for llvm_executionengine
21.129 [489/10/3600] Running utility command for ocaml_llvm_executionengine
22.214 [489/9/3601] Linking CXX executable bin/lli
22.968 [489/8/3602] Building AMDGPUGenAsmWriter.inc...
23.266 [489/7/3603] 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++ -DEXPENSIVE_CHECKS -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/lib/Target/RISCV -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/Target/RISCV -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/include -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include -U_GLIBCXX_DEBUG -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-clang-x86_64-expensive-checks-debian/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
/b/1/llvm-clang-x86_64-expensive-checks-debian/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-clang-x86_64-expensive-checks-debian/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-clang-x86_64-expensive-checks-debian/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-clang-x86_64-expensive-checks-debian/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-clang-x86_64-expensive-checks-debian/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-clang-x86_64-expensive-checks-debian/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.
23.741 [489/6/3604] Building AMDGPUGenGlobalISel.inc...
24.549 [489/5/3605] Building AMDGPUGenDAGISel.inc...
25.660 [489/4/3606] Building AMDGPUGenInstrInfo.inc...
27.065 [489/3/3607] Building AMDGPUGenRegisterBank.inc...
28.708 [489/2/3608] Building AMDGPUGenRegisterInfo.inc...
36.908 [489/1/3609] Building AMDGPUGenAsmMatcher.inc...
ninja: build stopped: subcommand failed.

```

</details>

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