[llvm] [VPlan] Add VPInst::getNumOperandsForOpcode, use to verify in ctor (NFC) (PR #142284)

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ayalz wrote:

> > > This can also be used in a follow-up to determine if a VPInstruction is masked based on the number of arguments.
> > 
> > 
> > BTW, masking could alternatively have an explicit API for setting and getting, possibly maintaining a pointer to a VPInstruction's mask or null when unmasked.
> 
> Yep, the WIP patch to use this for masking provides such an API, although based on the number of operands (https://github.com/llvm/llvm-project/pull/142285/files#diff-dabd8bf9956285f5ddab712af4e0494647a106234f07283b0bd9f8b4d9b887caR1039-R1073)
> 
> I think the mask operand still needs to be part of the operands of the VPInstruction, to make sure it is part of the def-use graph, to get updated automatically when the mask value gets RAUWs.

Agreed, the mask should be a regular operand, and in addition have a setter and getter to set and retrieve it.

https://github.com/llvm/llvm-project/pull/142284