[llvm] [TableGen] Implement getNamedOperandIdx with another table lookup. NFC. (PR #151116)

[Jay Foad via llvm-commits]

jayfoad wrote:

> WDYT? Is it wodth the addition 1/2 a KB?

I did consider this but my preference was to keep the code as simple as possible and non-branchy, even at the expense of slightly larger data.

If you really want to save space in this table, maybe we should represent it with an extra field in MCInstrDesc instead of a separate table? It looks like MCInstrDesc currently has a two byte hole that we could use.

https://github.com/llvm/llvm-project/pull/151116