[llvm] [SwitchLowering] Support merging 0 and power-of-2 case. (PR #139736)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Can you add the equivalent for GlobalISel too? It re-uses all of this code so it should be similar to the SDAG version you have here.

Thanks, added the missing code for GISel, just realized that it was only partially done


https://github.com/llvm/llvm-project/pull/139736