[llvm] [LLVM][MC][DecoderEmitter] Add support to specialize decoder per bitwidth (PR #154865)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-rhel` running on `ppc64le-clang-rhel-test` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/145/builds/9505

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
72.637 [602/19/6018] Building CXX object lib/Target/RISCV/CMakeFiles/LLVMRISCVCodeGen.dir/GISel/RISCVInstructionSelector.cpp.o
72.716 [601/19/6019] Linking CXX shared library lib/libLLVMRISCVCodeGen.so.22.0git
72.724 [600/19/6020] Creating library symlink lib/libLLVMRISCVCodeGen.so
75.808 [600/18/6021] Building CXX object lib/Target/AArch64/Disassembler/CMakeFiles/LLVMAArch64Disassembler.dir/AArch64Disassembler.cpp.o
75.852 [599/18/6022] Linking CXX shared library lib/libLLVMAArch64Disassembler.so.22.0git
75.858 [598/18/6023] Creating library symlink lib/libLLVMAArch64Disassembler.so
75.873 [596/19/6024] Linking CXX static library lib/libLLVMExegesisAArch64_static.a
75.899 [596/18/6025] Linking CXX shared library lib/libLLVMExegesisAArch64.so.22.0git
75.907 [595/18/6026] Creating library symlink lib/libLLVMExegesisAArch64.so
76.214 [595/17/6027] Building CXX object lib/Target/RISCV/Disassembler/CMakeFiles/LLVMRISCVDisassembler.dir/RISCVDisassembler.cpp.o
FAILED: lib/Target/RISCV/Disassembler/CMakeFiles/LLVMRISCVDisassembler.dir/RISCVDisassembler.cpp.o 
ccache /home/buildbots/llvm-external-buildbots/clang.19.1.7/bin/clang++ --gcc-toolchain=/gcc-toolchain/usr -DGTEST_HAS_RTTI=0 -DLLVM_EXPORTS -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib/Target/RISCV/Disassembler -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV/Disassembler -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/lib/Target/RISCV -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/build/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -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 -std=c++17 -fPIC  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT lib/Target/RISCV/Disassembler/CMakeFiles/LLVMRISCVDisassembler.dir/RISCVDisassembler.cpp.o -MF lib/Target/RISCV/Disassembler/CMakeFiles/LLVMRISCVDisassembler.dir/RISCVDisassembler.cpp.o.d -o lib/Target/RISCV/Disassembler/CMakeFiles/LLVMRISCVDisassembler.dir/RISCVDisassembler.cpp.o -c /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:705:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  705 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint16_t> = 16;
      |             ^~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:706:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  706 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
      |             ^~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:708:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  708 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 48;
      |             ^~~~~~
3 errors generated.
82.727 [595/16/6028] Building AMDGPUGenPostLegalizeGICombiner.inc...
83.115 [595/15/6029] Building AMDGPUGenRegBankGICombiner.inc...
83.480 [595/14/6030] Building AMDGPUGenMCPseudoLowering.inc...
84.637 [595/13/6031] Building AMDGPUGenPreLegalizeGICombiner.inc...
85.051 [595/12/6032] Building AMDGPUGenDisassemblerTables.inc...
85.288 [595/11/6033] Building AMDGPUGenSubtargetInfo.inc...
85.583 [595/10/6034] Building AMDGPUGenMCCodeEmitter.inc...
86.263 [595/9/6035] Building AMDGPUGenSearchableTables.inc...
86.829 [595/8/6036] Building AMDGPUGenCallingConv.inc...
91.115 [595/7/6037] Building AMDGPUGenAsmWriter.inc...
93.035 [595/6/6038] Building AMDGPUGenGlobalISel.inc...
94.678 [595/5/6039] Building AMDGPUGenInstrInfo.inc...
95.417 [595/4/6040] Building AMDGPUGenAsmMatcher.inc...
96.379 [595/3/6041] Building AMDGPUGenDAGISel.inc...
101.440 [595/2/6042] Building AMDGPUGenRegisterInfo.inc...
105.151 [595/1/6043] Building AMDGPUGenRegisterBank.inc...
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/154865