[VPlan] Mark FirstOrderRecurrenceSplice as not having side-effects.

Now that FOR exit and resume value creation is explicitly modeled in
VPlan (05e1b534, 07b33013) it doesn't depend on the first
order recurrence splice being preserved and it can now be marked as not
having side-effects. This allows removal of first-order-recurrence-splce
if the FOR is only used in the exit or as scalar ph resume value.