[AArch64] Assertion `PartVT.getFixedSizeInBits() > ValueSize && "lossy conversion of vector to scalar type"' failed.

Reproducer:
https://godbolt.org/z/7M3j7Txre

Backtrace:
```console
llc: /root/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:743: void getCopyToPartsVector(llvm::SelectionDAG&, const llvm::SDLoc&, llvm::SDValue, llvm::SDValue*, unsigned int, llvm::MVT, const llvm::Value*, std::optional): Assertion `PartVT.getFixedSizeInBits() > ValueSize && "lossy conversion of vector to scalar type"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/llc -o /app/output.s -x86-asm-syntax=intel
1. Running pass 'Function Pass Manager' on module ''.
2. Running pass 'AArch64 Instruction Selection' on function '@fpr_to_gpr'
#0 0x000000000423f888 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x423f888)
#1 0x000000000423c734 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#2 0x00007dcfc4442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#3 0x00007dcfc44969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#4 0x00007dcfc4442476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#5 0x00007dcfc44287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#6 0x00007dcfc442871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#7 0x00007dcfc4439e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#8 0x0000000003eec888 getCopyToParts(llvm::SelectionDAG&, llvm::SDLoc const&, llvm::SDValue, llvm::SDValue*, unsigned int, llvm::MVT, llvm::Value const*, std::optional, llvm::ISD::NodeType) SelectionDAGBuilder.cpp:0:0
#9 0x0000000003eecc39 llvm::RegsForValue::getCopyToRegs(llvm::SDValue, llvm::SelectionDAG&, llvm::SDLoc const&, llvm::SDValue&, llvm::SDValue*, llvm::Value const*, llvm::ISD::NodeType) const (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3eecc39)
#10 0x0000000003f13ecd llvm::SelectionDAGBuilder::visitInlineAsm(llvm::CallBase const&, llvm::BasicBlock const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3f13ecd)
#11 0x0000000003f308af llvm::SelectionDAGBuilder::visitCall(llvm::CallInst const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3f308af)
#12 0x0000000003f44c32 llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3f44c32)
#13 0x0000000003fd774d llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator_w_bits, false, true>, llvm::ilist_iterator_w_bits, false, true>, bool&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3fd774d)
#14 0x0000000003fd81b8 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3fd81b8)
#15 0x0000000003fd9cec llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3fd9cec)
#16 0x0000000003fc557f llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x3fc557f)
#17 0x0000000003117f29 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (.part.0) MachineFunctionPass.cpp:0:0
#18 0x000000000376a706 llvm::FPPassManager::runOnFunction(llvm::Function&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x376a706)
#19 0x000000000376aab1 llvm::FPPassManager::runOnModule(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x376aab1)
#20 0x000000000376b31f llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x376b31f)
#21 0x000000000090253d compileModule(char**, llvm::SmallVectorImpl&, llvm::LLVMContext&, std::__cxx11::basic_string, std::allocator>&) llc.cpp:0:0
#22 0x00000000007cbc1c main (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x7cbc1c)
#23 0x00007dcfc4429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#24 0x00007dcfc4429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#25 0x00000000008f7215 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/llc+0x8f7215)
Program terminated with signal: SIGSEGV
Compiler returned: 139
```

This comes from the following C reproducer:
```c
typedef unsigned char v16qi __attribute__((vector_size(16)));

void
fpr_to_gpr (v16qi q0)
{
register v16qi x0 asm ("x0");
x0 = q0;
asm volatile ("" :: "r" (x0));
}
```

Start with llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp, especially getCopyToPartsVector and the visitInlineAsm path shown by the backtrace. Reproduce the failure with the linked C example using llc for AArch64, then trace how the vector value is copied to registers. Done means the reproducer no longer triggers the assertion.

Written by the indexing model from the issue text.