[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 `premerge-monolithic-linux` running on `premerge-linux-1` while building `llvm` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/22597

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

```
Step 6 (build-unified-tree) failure: build (failure)
...
5.058 [3541/12/101] Linking CXX executable bin/find-all-symbols
5.058 [3541/11/102] Linking CXX executable bin/clang-reorder-fields
5.084 [3541/10/103] Linking CXX executable bin/clang-move
5.084 [3541/9/104] Linking CXX executable bin/pp-trace
5.085 [3541/8/105] Linking CXX executable bin/clang-include-fixer
5.098 [3541/7/106] Linking CXX executable bin/clang-refactor
5.114 [3541/6/107] Linking CXX executable bin/clang-query
5.139 [3541/5/108] Linking CXX executable bin/clang-include-cleaner
5.694 [3541/4/109] Linking CXX executable bin/clang-tidy
9.455 [3541/3/110] 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/build/buildbot/premerge-monolithic-linux/build/lib/Target/RISCV -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV -I/build/buildbot/premerge-monolithic-linux/build/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -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 /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5778:35: error: use of undeclared identifier 'M1VT'
    } else if (ContainerVT.bitsGT(M1VT) && isLowSourceShuffle(Mask, VLMAX)) {
                                  ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5778:69: error: use of undeclared identifier 'VLMAX'
    } else if (ContainerVT.bitsGT(M1VT) && isLowSourceShuffle(Mask, VLMAX)) {
                                                                    ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5784:24: error: use of undeclared identifier 'M1VT'
      EVT SubIndexVT = M1VT.changeVectorElementType(IndexVT.getScalarType());
                       ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5786:35: error: use of undeclared identifier 'M1VT'
          getDefaultScalableVLOps(M1VT, DL, DAG, Subtarget);
                                  ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5788:15: error: use of undeclared identifier 'M1VT'
              M1VT.getVectorMinNumElements();
              ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5792:43: error: use of undeclared identifier 'M1VT'
          DAG.getElementCount(DL, XLenVT, M1VT.getVectorElementCount());
                                          ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5799:38: error: use of undeclared identifier 'M1VT'
            DAG.getVectorIdxConstant(M1VT.getVectorMinNumElements() * i, DL);
                                     ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5801:53: error: use of undeclared identifier 'M1VT'
            DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, M1VT, V1, SubIdx);
                                                    ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5806:42: error: use of undeclared identifier 'M1VT'
            DAG.getNode(GatherVVOpc, DL, M1VT, SubV1, SubIndex,
                                         ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:5807:38: error: use of undeclared identifier 'M1VT'
                        DAG.getUNDEF(M1VT), InnerTrueMask, InnerVL);
                                     ^
10 errors generated.
12.195 [3541/2/111] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
13.567 [3541/1/112] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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