[llvm] [VPlan] Add helper to define CastInfo for casting to VP mixins (NFC) (PR #197494)

[Florian Hahn via llvm-commits]

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@@ -4044,34 +4044,32 @@ class LLVM_ABI_FOR_TEST VPScalarIVStepsRecipe : public VPRecipeWithIRFlags {
 #endif
 };
 
+/// CastInfo helper for casting from a recipe superclass (e.g. VPRecipeBase,
+/// VPUser) to a mixin class that is not part of the VPRecipeBase class
+/// hierarchy (e.g. VPPhiAccessors, VPIRMetadata).
+namespace vpdetail {
+template <class VPMixin, class From, class... RecipeTys>
+struct VPMixinCast
+    : public DefaultDoCastIfPossible<VPMixin *, From *,
+                                     VPMixinCast<VPMixin, From, RecipeTys...>> {
+  static_assert((std::is_base_of_v<VPMixin, RecipeTys> && ...),
+                "Each type in RecipeTys must derive from VPMixin");
+  static bool isPossible(From *R) { return isa<RecipeTys...>(R); }
+  static VPMixin *doCast(From *R) {
+    VPMixin *Out = nullptr;
+    (void)((Out = dyn_cast<RecipeTys>(R)) || ...);
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fhahn wrote:

The `dyn_casts` are from `VPRecipeBase*` to concrete recipe types, which go through a different path (recipe `classof` implementations)

https://github.com/llvm/llvm-project/pull/197494