[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:

Hello @arsenm ,

Thank you for the feedback!
Is the current version fine ?
I didn't want to make changes to other errors, so had created a separate if statement to handle zero arguments.

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