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

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> But it looks like handling duplicates requires quite a bit of extra complexity spread throughout

At the moment only the destructor needs to know that `ExpressionRecipes` contains duplicates when deleting them. What other complexity is required with regards to handling duplicates?

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