[SelectionDAG] Fix copyExtraInfo where new node has entry as operand (#149307)

Add special case handling where a new replacement node has the entry
node as an operand i.e. does not depend on any other nodes.

This can be observed with the existing X86/pcsections-atomics.ll test
case when targeting Haswell, where certain 128-bit atomics are
transformed into arch-specific instructions, with some operands having
no other dependencies.

Author: melver

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -13872,6 +13872,8 @@ void SelectionDAG::copyExtraInfo(SDNode *From, SDNode *To) {
     return;
   }
 
+  const SDNode *EntrySDN = getEntryNode().getNode();
+
   // We need to copy NodeExtraInfo to all _new_ nodes that are being introduced
   // through the replacement of From with To. Otherwise, replacements of a node
   // (From) with more complex nodes (To and its operands) may result in lost
@@ -13903,9 +13905,14 @@ void SelectionDAG::copyExtraInfo(SDNode *From, SDNode *To) {
       return true;
     if (!Visited.insert(N).second)
       return true;
-    if (getEntryNode().getNode() == N)
+    if (EntrySDN == N)
       return false;
     for (const SDValue &Op : N->op_values()) {
+      if (N == To && Op.getNode() == EntrySDN) {
+        // Special case: New node's operand is the entry node; just need to
+        // copy extra info to new node.
+        break;
+      }
       if (!Self(Self, Op.getNode()))
         return false;
     }