[llvm] [SelectionDAG] Fix null pointer dereference in resolveDanglingDebugInfo (PR #173500)

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MetalOxideSemi wrote:

> Testcase?

@arsenm Thank you for the review! I've added a regression test as requested. The test case (`test/CodeGen/Generic/selectiondag-dbgvalue-null-crash.ll`) reproduces the original crash scenario with an aggregate type containing empty structs and verifies that the fix prevents the null pointer dereference.

https://github.com/llvm/llvm-project/pull/173500