[llvm] [LV] Keep duplicate recipes in VPExpressionRecipe (PR #156976)

[Sander de Smalen via llvm-commits]

================
@@ -2801,21 +2800,36 @@ VPExpressionRecipe::VPExpressionRecipe(
   // create new temporary VPValues for all operands defined by a recipe outside
   // the expression. The original operands are added as operands of the
   // VPExpressionRecipe itself.
+
+  // This map caches the temporary placeholders so duplicates aren't created in
+  // the case that recipes share operands.
+  SmallMapVector<VPValue *, VPValue *, 4> OperandPlaceholders;
+
   for (auto *R : ExpressionRecipes) {
     for (const auto &[Idx, Op] : enumerate(R->operands())) {
       auto *Def = Op->getDefiningRecipe();
       if (Def && ExpressionRecipesAsSetOfUsers.contains(Def))
         continue;
       addOperand(Op);
-      LiveInPlaceholders.push_back(new VPValue());
-      R->setOperand(Idx, LiveInPlaceholders.back());
+      if (OperandPlaceholders.find(Op) == OperandPlaceholders.end())
----------------
sdesmalen-arm wrote:

There's no need to check for duplicates, you can just do:

```
LiveInPlaceholders.push_back(new VPValue());                                                    
OperandPlaceholders[Op] = LiveInPlaceholders.back();
```

https://github.com/llvm/llvm-project/pull/156976