[NFC][VPlan] Use VPUser to store block's predicate

[fhahn]

Thanks for taking a stab at this after D95382!

llvm/lib/Transforms/Vectorize/VPlan.h

428–429

The functions here are essentially the same as for CondBit, right? Could you factor out the logic into private helpers, that take a reference to VPUser?

[a.elovikov]

Move common implementation of get/set methods into helpers

[fhahn]

LGTM, thanks!