[llvm] [TableGen] Improvements to Named operands in InstrInfoEmitter (PR #124960)

[Jason Eckhardt via llvm-commits]

nvjle wrote:

Hi @jurahul ,

My downstream backend uses sentinel `OPERAND_LAST` (perhaps others do as well). It seems best to restore that line. It is pretty common for TableGen to emit sentinels in enums like this, and they can be useful.

https://github.com/llvm/llvm-project/pull/124960