[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:

Was the replacement the `Operand->isSubClassOf("PointerLikeRegClass")` check above? That's still hardcoding the class hierarchy set somewhere else. Should more directly avoid crashing on the getArg(0) 

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