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

[via llvm-commits]

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.

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