[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-linux-multistage` running on `ppc64le-clang-multistage-test` while building `llvm` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/12580

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

```
Step 4 (build stage 1) failure: 'ninja' (failure)
...
[5994/6500] Linking CXX shared library lib/libclangTidyGoogleModule.so.22.0git
[5995/6500] Creating library symlink lib/libclangTidyGoogleModule.so
[5996/6500] Linking CXX shared library lib/libclangTidyFuchsiaModule.so.22.0git
[5997/6500] Creating library symlink lib/libclangTidyFuchsiaModule.so
[5998/6500] Linking CXX shared library lib/libclangTidyBugproneModule.so.22.0git
[5999/6500] Creating library symlink lib/libclangTidyBugproneModule.so
[6000/6500] Building AMDGPUGenDAGISel.inc...
[6001/6500] Building AMDGPUGenAsmWriter.inc...
[6002/6500] Linking CXX shared library lib/libclangTidyCERTModule.so.22.0git
[6003/6500] 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 /usr/lib64/ccache/c++ -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-multistage-test/clang-ppc64le-multistage/stage1/lib/Target/RISCV/Disassembler -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV/Disassembler -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/lib/Target/RISCV -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/stage1/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/include -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-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment -Wno-misleading-indentation -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-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:705:13: error: explicit template specialization cannot have a storage class
 template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint16_t> = 16;
             ^~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:706:13: error: explicit template specialization cannot have a storage class
 template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
             ^~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:708:13: error: explicit template specialization cannot have a storage class
 template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 48;
             ^~~~~~
[6004/6500] Creating library symlink lib/libclangTidyCERTModule.so
[6005/6500] Linking CXX shared library lib/libclangTidyModernizeModule.so.22.0git
[6006/6500] Building AMDGPUGenGlobalISel.inc...
[6007/6500] Building AMDGPUGenAsmMatcher.inc...
[6008/6500] Building AMDGPUGenInstrInfo.inc...
[6009/6500] Building CXX object lib/Target/AArch64/Disassembler/CMakeFiles/LLVMAArch64Disassembler.dir/AArch64Disassembler.cpp.o
[6010/6500] Building AMDGPUGenRegisterInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

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