[llvm] [InstCombine] Fold `switch(zext/sext(X))` into `switch(X)` (PR #76988)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Why is this in SimplifyCFG? This seems like an extension of the existing switch width fold in InstCombine:
> 
> https://github.com/llvm/llvm-project/blob/8b2bdfbca7c1db272e4e703445f5626b4bc4b9d3/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp#L3224

Done.

https://github.com/llvm/llvm-project/pull/76988