[VPlan] Allow sinking of instructions with no defs

We started seeing new failure after D142886. Looks like it enabled new cases and we hit an assert:
assert(Current->getNumDefinedValues() == 1 &&
           "only recipes with a single defined value expected");

 When we do instruction sinking for the first order recurrence we hit an assert if instruction doesn't have single def. In case instruction doesn't produce any new def there is no new users and nothing to sink.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D151204