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

[Luke Lau 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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lukel97 wrote:

I think `VPWidenIntrinsic` also could use it, although it calls `::getScalarType` `::getResultType`. But agreed, I think it can be done as a follow up. It would be nice to get this in to eventually unblock #129508 and #118638

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