G1: policy for how many old regions to add to the CSet (when young gen is fixed) is broken

Unfortunately, with the fix for:

7050392: G1: Introduce flag to generate a log of the G1 ergonomic decisions

I accidentally broke the policy that decides how many old regions to add to the CSet when the young gen is fixed. The original code was:

      should_continue =
        ( hr != NULL) &&
        ( (adaptive_young_list_length()) ? time_remaining_ms > 0.0
          : _collection_set_size < _young_list_fixed_length );

In the fix for 7050392 I refactored it to be able to emit the correct ergo policy output:

      should_continue = true;
      if (hr == NULL) {
        // No need for an ergo verbose message here,
        // getNextMarkRegion() does this when it returns NULL.
        should_continue = false;
      } else {
        if (adaptive_young_list_length()) {
          if (time_remaining_ms < 0.0) {
            ergo_verbose1(ErgoCSetConstruction, ...);
            should_continue = false;
          }
        } else {
          if (_collection_set_size < _young_list_fixed_length) {
            ergo_verbose2(ErgoCSetConstruction, ...);
            should_continue = false;
          }
        }
      }

and unfortunately I didn't negate the _collection_set_size < _young_list_fixed_length condition. The intention of this code is: if hr != NULL (which means: we've just found and added an old region to the CSet) and !adaptive_young_list_length() (which means: the young gen size is fixed), we should carry on adding old regions to the CSet until the CSet length (_collection_set_size) reaches the fixed young list target length (_young_list_fixed_length). So, should_continue should be set to false when _collection_set size >= _young_list_fixed_length, not when _collection_set_size < _young_list_fixed_length.

As is, the code can do the wrong thing in a couple of ways:

- Only add a single old region to the CSet and exit (even though we can add many more).
- If the young length is too small and there's space for only one old region, we'll reach the target after adding a single old region to the CSet and we'll then end up adding the remaining old regions irrespective of how long the CSet will be.