[llvm] [VPlan] Introduce VPInstructionWithType, use instead of VPScalarCast(NFC) (PR #129706)

[via llvm-commits]

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@@ -1024,6 +1023,53 @@ class VPInstruction : public VPRecipeWithIRFlags,
   StringRef getName() const { return Name; }
 };
 
+/// A specialization of VPInstruction augmenting it with a dedicated result
+/// type, to be used when the opcode and operands of the VPInstruction don't
+/// directly determine the result type.
+class VPInstructionWithType : public VPInstruction {
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ayalz wrote:

A recipe can "have" a single def, and support the interface of VPValue, by inheriting from it.
A (single def) recipe can have IRflags, by being VPRecipeWithFlags.
A (single def with IRFlags) recipe can have an opcode, by being a VPInstruction.
A (single def with IRFlags with opcode) recipe can have a Type, by being a VPInstructionWithType.
Would using multiple inheritance be better to cope with desired combinations of independent "with's", where IRFlags, opcode, Type, are provided as base classes? In particular, allowing recipes to have Type w/o having IRFlags.

https://github.com/llvm/llvm-project/pull/129706