With CL 190599, along with what we do in greyobject, we ensure
    that we only mark allocated heap objects. As a result we can be
    more strict in GC:
    
    - Enable "sweep increased allocation count" check, which checks
      that the number of mark bits set are no more than the number of
      allocation bits.
    
    - Enable invalid pointer check on heap scan. We only trace
      allocated heap objects, which should not contain invalid
      pointer.
    
    This also makes the libgo runtime more convergent with the gc
    runtime.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/190797

From-SVN: r274678