In tiered allocation, cut non-promising shrinking tries

In tiered allocation, cut non-promising shrinking tries

The heuristics for tiered allocation has been improved in that it
chooses to shrink such a resource next where by shrinking only this
resource a valid allocation can be made, if such a resource exists.
In order to decide whether such a resource exists, all potential
sizes of this resource are verified. We can improve performance by
cutting, as soon a potential allocation of an instance is no longer
blocked on this resource.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>