[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 `openmp-s390x-linux` running on `systemz-1` while building `llvm` at step 5 "compile-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/88/builds/15612

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

```
Step 5 (compile-openmp) failure: build (failure)
...
206.068 [2969/4/2501] Building CXX object lib/Target/Lanai/CMakeFiles/LLVMLanaiCodeGen.dir/LanaiTargetObjectFile.cpp.o
206.089 [2968/4/2502] Building CXX object lib/Target/Lanai/AsmParser/CMakeFiles/LLVMLanaiAsmParser.dir/LanaiAsmParser.cpp.o
206.119 [2967/4/2503] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiAsmBackend.cpp.o
206.143 [2966/4/2504] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiELFObjectWriter.cpp.o
206.169 [2965/4/2505] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiInstPrinter.cpp.o
206.186 [2964/4/2506] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiMCAsmInfo.cpp.o
206.209 [2963/4/2507] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiMCCodeEmitter.cpp.o
206.232 [2962/4/2508] Building CXX object lib/Target/Lanai/MCTargetDesc/CMakeFiles/LLVMLanaiDesc.dir/LanaiMCTargetDesc.cpp.o
206.841 [2961/4/2509] Building LoongArchGenAsmWriter.inc...
208.001 [2960/4/2510] Building CXX object lib/Target/AMDGPU/Disassembler/CMakeFiles/LLVMAMDGPUDisassembler.dir/AMDGPUDisassembler.cpp.o
FAILED: lib/Target/AMDGPU/Disassembler/CMakeFiles/LLVMAMDGPUDisassembler.dir/AMDGPUDisassembler.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros /usr/bin/ccache /usr/bin/c++ -DGTEST_HAS_RTTI=0 -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/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/lib/Target/AMDGPU/Disassembler -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/lib/Target/AMDGPU -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/include -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/include -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/lib/Target/AMDGPU/Disassembler/.. -I/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/.. -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -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 -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/AMDGPU/Disassembler/CMakeFiles/LLVMAMDGPUDisassembler.dir/AMDGPUDisassembler.cpp.o -MF lib/Target/AMDGPU/Disassembler/CMakeFiles/LLVMAMDGPUDisassembler.dir/AMDGPUDisassembler.cpp.o.d -o lib/Target/AMDGPU/Disassembler/CMakeFiles/LLVMAMDGPUDisassembler.dir/AMDGPUDisassembler.cpp.o -c /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp:451: error: explicit template specialization cannot have a storage class
  451 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
      | 
/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp:452: error: explicit template specialization cannot have a storage class
  452 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 64;
      | 
/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp:454: error: explicit template specialization cannot have a storage class
  454 | static constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<96>> = 96;
      | 
/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp:456: error: explicit template specialization cannot have a storage class
  456 | static constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<128>> = 128;
      | 
208.189 [2960/3/2511] Building CXX object lib/Target/Hexagon/Disassembler/CMakeFiles/LLVMHexagonDisassembler.dir/HexagonDisassembler.cpp.o
208.364 [2960/2/2512] Building LoongArchGenDAGISel.inc...
209.374 [2960/1/2513] Building CXX object lib/Target/Lanai/Disassembler/CMakeFiles/LLVMLanaiDisassembler.dir/LanaiDisassembler.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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