[VPlan] Properly retain flags when cloning VPReplicateRecipe.

This makes sure the correct flags are used for the clone (i.e. the ones
present on the recipe), instead of the ones on the original IR
instruction.

At the moment, this should not change anything, as flags of replicate
recipe should not be dropped before they are cloned at the moment. But
that will change in a follow-up patch.