postfix `++` on bitfield segfaults the compiler

[drpriver]

$ ldc2 --version
LDC - the LLVM D compiler (1.40.0):
  based on DMD v2.110.0 and LLVM 19.1.3
  built with LDC - the LLVM D compiler (1.39.0)
  Default target: arm64-apple-darwin23.6.0
  Host CPU: apple-m1
  http://dlang.org - http://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv       - SPIR-V Logical
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
    xtensa      - Xtensa 32

Compiling this C file causes a segfault:

// repro.c
void foo(void){
    struct Foo {
        unsigned count:31;
    };
    struct Foo f = {0};
    f.count++;
}

Compiler output:

Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  ldc2                     0x0000000105449f4c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  ldc2                     0x0000000105448170 llvm::sys::RunSignalHandlers() + 64
2  ldc2                     0x000000010544a5a8 SignalHandler(int) + 292
3  libsystem_platform.dylib 0x0000000181426584 _sigtramp + 56
4  ldc2                     0x0000000105829bcc ToElemVisitor::visit(PostExp*) + 372
5  ldc2                     0x0000000105819be4 ToIRVisitor::visit(ExpStatement*) + 448
6  ldc2                     0x0000000105819dfc ToIRVisitor::visit(CompoundStatement*) + 308
7  ldc2                     0x0000000105819dfc ToIRVisitor::visit(CompoundStatement*) + 308
8  ldc2                     0x00000001057d86d8 DtoDefineFunction(FuncDeclaration*, bool) + 5988
9  ldc2                     0x00000001057c7870 CodegenVisitor::visit(AttribDeclaration*) + 80
10 ldc2                     0x00000001057e8f48 codegenModule(IRState*, Module*) + 3140
11 ldc2                     0x000000010586117c ldc::CodeGenerator::emit(Module*) + 216
12 ldc2                     0x000000010587aa78 codegenModules(Array<Module*>&) + 976
13 ldc2                     0x000000010578b424 mars_tryMain(Param&, Array<char const*>&) + 1528
14 ldc2                     0x0000000105a0efbc _D2rt6dmain212_d_run_main2UAAamPUQgZiZ6runAllMFZv + 116
15 ldc2                     0x0000000105a0eae0 _d_run_main + 144
16 ldc2                     0x0000000105877c0c main + 744
17 dyld                     0x000000018106b154 start + 2476