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

[Florian Hahn via llvm-commits]

================
@@ -2801,21 +2800,31 @@ 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.
+
+  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());
+      VPValue *Tmp = new VPValue();
+      OperandPlaceholders[Op] = Tmp;
+      LiveInPlaceholders.push_back(Tmp);
     }
   }
+
+  for (auto *R : ExpressionRecipes)
----------------
fhahn wrote:

This subtly changes the behavior; we could have the same external operand used multiple times, and we will have multiple entries for them in LiveInPlaceholders, but only use the first of them.

I'm not sure if there's a way around this inconsistency?

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