Impossible reg-to-reg copy when returning a zeroed 40-bit struct

[shepmaster]

target datalayout = "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8"
target triple = "avr-unknown-unknown"

define hidden { i8, i32 } @"extract_sign"(i32 %self) unnamed_addr addrspace(1) {
start:
  ret { i8, i32 } zeroinitializer
}

llc -march=avr -mcpu=atmega328p -filetype=obj ./small.ll

Impossible reg-to-reg copy
UNREACHABLE executed at /Users/shep/Projects/rust/src/llvm-project/llvm/lib/Target/AVR/AVRInstrInfo.cpp:74!
Stack dump:
0.	Program arguments: llc -march=avr -mcpu=atmega328p -filetype=obj ./small.ll
1.	Running pass 'Function Pass Manager' on module './small.ll'.
2.	Running pass 'Post-RA pseudo instruction expansion pass' on function '@extract_sign'
0  libLLVM.dylib            0x000000011694ca58 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  libLLVM.dylib            0x000000011694bae5 llvm::sys::RunSignalHandlers() + 85
2  libLLVM.dylib            0x000000011694d036 SignalHandler(int) + 262
3  libsystem_platform.dylib 0x00007fff6e8af5fd _sigtramp + 29
4  libsystem_platform.dylib 0x000000000000004f _sigtramp + 18446603338661562991
5  libsystem_c.dylib        0x00007fff6e785808 abort + 120
6  libLLVM.dylib            0x0000000118d8461e llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 462
7  libLLVM.dylib            0x0000000118c3e161 llvm::AVRInstrInfo::copyPhysReg(llvm::MachineBasicBlock&, llvm::MachineInstrBundleIterator<llvm::MachineInstr, false>, llvm::DebugLoc const&, llvm::MCRegister, llvm::MCRegister, bool) const + 993
8  libLLVM.dylib            0x0000000116b74b1e (anonymous namespace)::ExpandPostRA::runOnMachineFunction(llvm::MachineFunction&) + 2894
9  libLLVM.dylib            0x0000000116c5bbbd llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 349
10 libLLVM.dylib            0x0000000116a7cc08 llvm::FPPassManager::runOnFunction(llvm::Function&) + 1064
11 libLLVM.dylib            0x0000000116a7cec8 llvm::FPPassManager::runOnModule(llvm::Module&) + 72
12 libLLVM.dylib            0x0000000116a7d3be llvm::legacy::PassManagerImpl::run(llvm::Module&) + 1022
13 llc                      0x000000010fbbbebd compileModule(char**, llvm::LLVMContext&) + 8909
14 llc                      0x000000010fbb970c main + 1516
15 libdyld.dylib            0x00007fff6e6b6cc9 start + 1

% llvm-config --version
10.0.1-rust-dev

Rust commit f4fbb93