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

[Amara Emerson via llvm-commits]

https://github.com/aemerson 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.

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