[SelectionDAG] Fix null pointer dereference in resolveDanglingDebugInfo (#174341)

## Summary
Fix null pointer dereference in
`SelectionDAGBuilder::resolveDanglingDebugInfo`.

## Problem
`Val.getNode()->getIROrder()` is called before checking if
`Val.getNode()` is null, causing crashes when compiling code with debug
info that contains aggregate constants with nested empty structs.

## Solution
Move the `ValSDNodeOrder` declaration inside the `if (Val.getNode())`
block.

## Test Case
Reproduces with aggregate types containing nested empty structs:
```llvm
%3 = insertvalue { { i1, {} }, ptr, { { {} }, { {} } }, i64 } 
     { { i1, {} } zeroinitializer, ptr null, { { {} }, { {} } } zeroinitializer, i64 2 }, 
     ptr %2, 1, !dbg !893

## Crash stack
0.      Program arguments: llc-20 -O3 -mcpu=native -relocation-model=pic -filetype=obj /cloudide/workspace/temp/sf.ll -o /dev/null
1.      Running pass 'Function Pass Manager' on module '/cloudide/workspace/temp/sf.ll'.
2.      Running pass 'X86 DAG->DAG Instruction Selection' on function '@filter_create'
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  libLLVM.so.20.1 0x00007ff87ebbdf86 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 54
1  libLLVM.so.20.1 0x00007ff87ebbbb90 llvm::sys::RunSignalHandlers() + 80
2  libLLVM.so.20.1 0x00007ff87ebbe640
3  libpthread.so.0 0x00007ff87db79140
4  libLLVM.so.20.1 0x00007ff87f3fd2ff llvm::SelectionDAGBuilder::resolveDanglingDebugInfo(llvm::Value const*, llvm::SDValue) + 303
5  libLLVM.so.20.1 0x00007ff87f3fda5e llvm::SelectionDAGBuilder::getValue(llvm::Value const*) + 142
6  libLLVM.so.20.1 0x00007ff87f3fe79f llvm::SelectionDAGBuilder::getValueImpl(llvm::Value const*) + 3343
7  libLLVM.so.20.1 0x00007ff87f3fda34 llvm::SelectionDAGBuilder::getValue(llvm::Value const*) + 100
8  libLLVM.so.20.1 0x00007ff87f3fc1ab llvm::SelectionDAGBuilder::visitInsertValue(llvm::InsertValueInst const&) + 603
9  libLLVM.so.20.1 0x00007ff87f3eeaf7 llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) + 327
10 libLLVM.so.20.1 0x00007ff87f4904b8 llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator_w_bits<llvm::ilist_detail::node_options<llvm::Instruction, false, false, void, true, llvm::BasicBlock>, false, true>, llvm::ilist_iterator_w_bits<llvm::ilist_detail::node_options<llvm::Instruction, false, false, void, true, llvm::BasicBlock>, false, true>, bool&) + 72
11 libLLVM.so.20.1 0x00007ff87f490304 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) + 5956
12 libLLVM.so.20.1 0x00007ff87f48e2b4 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) + 372
13 libLLVM.so.20.1 0x00007ff87f48c689 llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) + 169
14 libLLVM.so.20.1 0x00007ff87efb8e32 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 610
15 libLLVM.so.20.1 0x00007ff87ed104be llvm::FPPassManager::runOnFunction(llvm::Function&) + 638
16 libLLVM.so.20.1 0x00007ff87ed15ff3 llvm::FPPassManager::runOnModule(llvm::Module&) + 51
17 libLLVM.so.20.1 0x00007ff87ed10c11 llvm::legacy::PassManagerImpl::run(llvm::Module&) + 1105
18 llc-20          0x000055972ce77dc1 main + 9649
19 libc.so.6       0x00007ff87d68ad7a __libc_start_main + 234
20 llc-20          0x000055972ce7247a _start + 42
```

## Testing

Added regression tests in:
- `CodeGen/X86/selectiondag-dbgvalue-null-crash.ll`
- `CodeGen/AArch64/selectiondag-dbgvalue-null-crash.ll`

**Note:** Tests appear to expose deeper issues in DWARF generation on
certain targets (Darwin targets for example) that require further
investigation.

## Related PRs

This supersedes:
- #173500 - Initial fix, reverted due to test failures on Darwin and
other platforms
- #173836 - Second attempt with `UNSUPPORTED: system-darwin`, still
failed on some targets