[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 `lld-x86_64-ubuntu-fast` running on `as-builder-4` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/33/builds/22152

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
36.769 [484/10/3548] Linking CXX static library lib/libLLVMRISCVCodeGen.a
36.903 [484/9/3549] Building AMDGPUGenAsmWriter.inc...
37.611 [484/8/3550] Building AMDGPUGenGlobalISel.inc...
37.742 [484/7/3551] Building AMDGPUGenAsmMatcher.inc...
38.804 [484/6/3552] Building AMDGPUGenInstrInfo.inc...
39.558 [484/5/3553] Building AMDGPUGenDAGISel.inc...
40.189 [484/4/3554] Building CXX object lib/Target/AArch64/Disassembler/CMakeFiles/LLVMAArch64Disassembler.dir/AArch64Disassembler.cpp.o
40.258 [483/4/3555] Linking CXX static library lib/libLLVMAArch64Disassembler.a
40.322 [482/4/3556] Linking CXX static library lib/libLLVMExegesisAArch64.a
40.385 [482/3/3557] 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_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/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/lib/Target/RISCV/Disassembler -I/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV/Disassembler -I/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV -I/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/lib/Target/RISCV -I/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/include -I/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/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-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 -std=c++17 -fvisibility=hidden  -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/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:705:13: error: explicit template specialization cannot have a storage class
  705 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint16_t> = 16;
      |             ^~~~~~
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:706:13: error: explicit template specialization cannot have a storage class
  706 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
      |             ^~~~~~
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp:708:13: error: explicit template specialization cannot have a storage class
  708 | template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 48;
      |             ^~~~~~
42.094 [482/2/3558] Building AMDGPUGenRegisterBank.inc...
42.703 [482/1/3559] Building AMDGPUGenRegisterInfo.inc...
ninja: build stopped: subcommand failed.

```

</details>

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