[llvm] [SimplifyCFG] Replace unreachable switch lookup table holes with poison (PR #94990)

[via llvm-commits]

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@@ -6322,6 +6333,9 @@ SwitchLookupTable::SwitchLookupTable(
       if (!ConstVal) {
         // This is an undef. We could deal with it, but undefs in lookup tables
         // are very seldom. It's probably not worth the additional complexity.
+        // TODO: In switches with holes and an unreachable default branch, this
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DianQK wrote:

Do you want to finish this TODO in this PR? If not, could you change the poison values back to the first value? I'm concerned this might cause some regressions.

https://github.com/llvm/llvm-project/pull/94990