[llvm] [SimplifyCFG] When only one case value is missing, replace default with that case (PR #76669)

[Quentin Dian via llvm-commits]

DianQK wrote:

> I'd suggest adding a test variant that allows conversion to lookup table, as that's where we expect to see the main improvement.

I added two test variants.

https://github.com/llvm/llvm-project/pull/76669