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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

Error messages should start with a lowercase (although the other cases here also violate this policy). The phrasing is also an inconsistent style. Avoid referring to "This" and "it"

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