The assertion, as currently constructed is too strong
in the presence of CMS, even without +ExplicitGCInvokesConcurrent.

Need a code-audit to make sure that the intended invariant was
not used anywhere for correctness before relaxing the assertion.
Or will need to modify the code to respect the intended invariant.

Tjis bug also covers the assertion violations observed in the
recent nightly tests with +ExplicitGCInvokesConcurrent:

  assert(_gc_count_before < gch->total_collections(), "Consistency");

AND

  assert(!full || (gc_count_before >= full_gc_count_before), "Invariant");