[LegalizeDAG] Return after replacing ISD::POISON with ISD::UNDEF. (#173173)

We already replaced the node, we shouldn't run the rest of the code that
still uses the old node.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -993,18 +993,18 @@ void SelectionDAGLegalize::LegalizeOp(SDNode *Node) {
   TargetLowering::LegalizeAction Action = TargetLowering::Legal;
   bool SimpleFinishLegalizing = true;
   switch (Node->getOpcode()) {
-  // TODO: Currently, POISON is being lowered to UNDEF here. However, there is
-  // an open concern that this transformation may not be ideal, as targets
-  // should ideally handle POISON directly. Changing this behavior would require
-  // adding support for POISON in TableGen, which is a large change.
-  // Additionally, many existing test cases rely on the current behavior (e.g.,
-  // llvm/test/CodeGen/PowerPC/vec_shuffle.ll). A broader discussion and
-  // incremental changes might be needed to properly
-  // support POISON without breaking existing targets and tests.
   case ISD::POISON: {
+    // TODO: Currently, POISON is being lowered to UNDEF here. However, there is
+    // an open concern that this transformation may not be ideal, as targets
+    // should ideally handle POISON directly. Changing this behavior would
+    // require adding support for POISON in TableGen, which is a large change.
+    // Additionally, many existing test cases rely on the current behavior
+    // (e.g., llvm/test/CodeGen/PowerPC/vec_shuffle.ll). A broader discussion
+    // and incremental changes might be needed to properly support POISON
+    // without breaking existing targets and tests.
     SDValue UndefNode = DAG.getUNDEF(Node->getValueType(0));
     ReplaceNode(Node, UndefNode.getNode());
-    break;
+    return;
   }
   case ISD::INTRINSIC_W_CHAIN:
   case ISD::INTRINSIC_WO_CHAIN: