[llvm] [X86][GlobalISel] Enable G_BRJT (PR #81811)

[Rémi SEGARD via llvm-commits]

RemiSEGARD wrote:

@e-kud @topperc Thanks for the feedback! I like the idea of lowering G_BRJT to simpler instructions during the legalizer. I'm currently very unfamiliar with SelectionDag so i was just trying to replicate the instructions it was selecting.
I will look into your suggestions and see what I can do!

https://github.com/llvm/llvm-project/pull/81811