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

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM; the test consideration was just to see if there was an obvious way to preserve existing coverage, it sounds like that's not easy or maybe possible, so never mind.

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