[llvm] [ConstraintElim] Handle switch cases with the same destination (PR #76928)

[Nikita Popov via llvm-commits]

nikic wrote:

@dtcxzyw Can we solve this by folding `switch(zext(x))` to `switch(x)` instead? That seems like a generally beneficial transform...

https://github.com/llvm/llvm-project/pull/76928