Support big-step shrinking in tiered allocation

Support big-step shrinking in tiered allocation

In tiered allocation, if by shrinking only a single resource a valid
allocation can be found, shrinking is bound to shrink on this resource.
Of course, after shrinking that resource a little bit without finding
an allocation, this property is still valid. So we can as well shrink
on that resource as far as needed to get a valid allocation.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>