[llvm] [LV] Make having flags (FMFs etc) not require inheriting VPSingleDefRecipe (PR #114972)

[Benjamin Maxwell via llvm-commits]

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@@ -803,6 +805,13 @@ class VPRecipeBase : public ilist_node_with_parent<VPRecipeBase, VPBasicBlock>,
   /// Returns the debug location of the recipe.
   DebugLoc getDebugLoc() const { return DL; }
 
+  /// Returns the IR flags for the recipe.
+  virtual const VPRecipeIRFlags *getIRFlags() const { return nullptr; }
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MacDue wrote:

I'll see... :thinking:  I'm not convinced that would be a better solution. It makes it required for recipes to inherit from `VPRecipeIRFlags` (so `VPRecipeBase*` is castable to `VPRecipeIRFlags*`), and add the additional maintenance keeping the list of classes in the `classof` -- without a `getIRFlags()` on the base). 

Given there's only one place that did a VPValue -> IRFlags cast before (the rest easily replaced with `.getIRFlags()`), it does not seem like a critical feature.

https://github.com/llvm/llvm-project/pull/114972