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

[Quentin Dian via llvm-commits]

DianQK wrote:

> Emm, this patch seems to increase the inline cost estimation of switch insts. Not sure it is a regression. Please have a look at `bench/hermes/optimized/SourceMgr.cpp.ll`. https://github.com/dtcxzyw/llvm-opt-benchmark/pull/80/files

I'm not sure this is a regression. But I don't think it would prevent merging this PR. To avoid this issue being lost, I created #76772 to track it.

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