[VPlan] Move usesScalars/onlyFirstLaneUsed to VPUser.

[fhahn]

Summary

Those helpers model properties of a user and they should also be
available to non-recipe users. This will be used in D123537 for a new
exit value user.

[fhahn]

Remove casts from VPUser -> VPRecipeBase before accessing usesScalars/onlyFirstLaneUsed.

[fhahn]

Ping :)

[Ayal]

Looks good! Minor documentation update nits.

llvm/lib/Transforms/Vectorize/VPlanValue.h

291

nit: "recipe" >> "VPUser"?

299

ditto

[fhahn]

Address comments & aebase before landing.

llvm/lib/Transforms/Vectorize/VPlan.h

public:
 
   /// Returns true if the recipe may write to memory.
   bool mayWriteToMemory() const;
 
   /// Returns true if the recipe may read from or write to memory.
   bool mayReadOrWriteMemory() const {
     return mayReadFromMemory() || mayWriteToMemory();
   }
-
-  /// Returns true if the recipe only uses the first lane of operand \p Op.
-  /// Conservatively returns false.
-  virtual bool onlyFirstLaneUsed(const VPValue *Op) const {
-    assert(is_contained(operands(), Op) &&
-           "Op must be an operand of the recipe");
-    return false;
-  }
-
-  /// Returns true if the recipe uses scalars of operand \p Op. Conservatively
-  /// returns if only first (scalar) lane is used, as default.
-  virtual bool usesScalars(const VPValue *Op) const {
-    assert(is_contained(operands(), Op) &&
-           "Op must be an operand of the recipe");
-    return onlyFirstLaneUsed(Op);
-  }
 };
 
 inline bool VPUser::classof(const VPDef *Def) {
   return Def->getVPDefID() == VPRecipeBase::VPInstructionSC ||
          Def->getVPDefID() == VPRecipeBase::VPWidenSC ||
          Def->getVPDefID() == VPRecipeBase::VPWidenCallSC ||
          Def->getVPDefID() == VPRecipeBase::VPWidenSelectSC ||
          Def->getVPDefID() == VPRecipeBase::VPWidenGEPSC ||

llvm/lib/Transforms/Vectorize/VPlanValue.h

public:
   const_operand_iterator op_end() const { return Operands.end(); }
   operand_range operands() { return operand_range(op_begin(), op_end()); }
   const_operand_range operands() const {
     return const_operand_range(op_begin(), op_end());
   }
 
   /// Method to support type inquiry through isa, cast, and dyn_cast.
   static inline bool classof(const VPDef *Recipe);
+
+  /// Returns true if the recipe uses scalars of operand \p Op. Conservatively
+  /// returns if only first (scalar) lane is used, as default.
+  virtual bool usesScalars(const VPValue *Op) const {
+    assert(is_contained(operands(), Op) &&
+           "Op must be an operand of the recipe");
+    return onlyFirstLaneUsed(Op);
+  }
+
+  /// Returns true if the recipe only uses the first lane of operand \p Op.
+  /// Conservatively returns false.
+  virtual bool onlyFirstLaneUsed(const VPValue *Op) const {
+    assert(is_contained(operands(), Op) &&
+           "Op must be an operand of the recipe");
+    return false;
+  }
 };
 
 /// This class augments a recipe with a set of VPValues defined by the recipe.
 /// It allows recipes to define zero, one or multiple VPValues. A VPDef owns
 /// the VPValues it defines and is responsible for deleting its defined values.
 /// Single-value VPDefs that also inherit from VPValue must make sure to inherit
 /// from VPDef before VPValue.
 class VPDef {