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

[via llvm-commits]

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@@ -6266,7 +6266,8 @@ SwitchLookupTable::SwitchLookupTable(
   assert(Values.size() && "Can't build lookup table without values!");
   assert(TableSize >= Values.size() && "Can't fit values in table!");
 
-  // If all values in the table are equal, this is that value.
+  // If all values in the table are equal, ignoring any values that are poison,
+  // this is that value.
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DianQK wrote:

This is an internal scenario, so for me, adding a comment in the corresponding code is sufficient.

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