[llvm] [Tablegen] Don't emit decoder tables with islands larger than 64 bits (PR #179651)

[Sam Elliott via llvm-commits]

lenary wrote:

Please can you add a self-contained `.td` file as a llvm-lit unit test, to show this working? There should be others in `llvm/test/TableGen/DecoderEmitter` which you should be able to adapt.

https://github.com/llvm/llvm-project/pull/179651