[llvm] [TableGen] Gracefully error out in ParseTreePattern when DAG has zero operands so that llvm-tblgen doesn't crash (PR #161417)

[Prerona Chaudhuri via llvm-commits]

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pchaudhuri-nv wrote:

I think we would still need changes to UpdateNodeTypeFromInst, because after returning nullptr from ParseTreePattern, we invoke ParseOnePattern which invokes UpdateNodeTypeFromInst

https://github.com/llvm/llvm-project/pull/161417