[llvm] [SimplifyCFG][ConstantMerge] Test non-table cases ahead of a switch's lookup table (PR #223255)

[Nazar Mokrynskyi via llvm-commits]

nazar-pc wrote:

I can split two commits into separate PRs after review, but it makes the most logical sense to land them together.

https://github.com/llvm/llvm-project/pull/223255