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

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``````````bash
git-clang-format --diff f8d270474c14c6705c77971494505dbe4b6d55ae 65dd6538d4cda2895dfc1e2b9259ac29d5a44c3a --extensions cpp -- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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View the diff from clang-format here.
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``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index b934ed8576..1f265d648f 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6534,21 +6534,21 @@ SwitchLookupTable::SwitchLookupTable(
     if (SingleValue && !isa<PoisonValue>(CaseRes) && CaseRes != SingleValue)
       SingleValue = isa<PoisonValue>(SingleValue) ? CaseRes : nullptr;
 
-  // Fill in any holes in the table with the default result.
-  if (Values.size() < TableSize) {
-    assert(DefaultValue &&
-           "Need a default value to fill the lookup table holes.");
-    assert(DefaultValue->getType() == ValueType);
-    for (uint64_t I = 0; I < TableSize; ++I) {
-      if (!TableContents[I])
-        TableContents[I] = DefaultValue;
-    }
+    // Fill in any holes in the table with the default result.
+    if (Values.size() < TableSize) {
+      assert(DefaultValue &&
+             "Need a default value to fill the lookup table holes.");
+      assert(DefaultValue->getType() == ValueType);
+      for (uint64_t I = 0; I < TableSize; ++I) {
+        if (!TableContents[I])
+          TableContents[I] = DefaultValue;
+      }
 
-    // If the default value is poison, all the holes are poison.
-    bool DefaultValueIsPoison = isa<PoisonValue>(DefaultValue);
+      // If the default value is poison, all the holes are poison.
+      bool DefaultValueIsPoison = isa<PoisonValue>(DefaultValue);
 
-    if (DefaultValue != SingleValue && !DefaultValueIsPoison)
-      SingleValue = nullptr;
+      if (DefaultValue != SingleValue && !DefaultValueIsPoison)
+        SingleValue = nullptr;
   }
 
   // If each element in the table contains the same value, we only need to store

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