code: public IsNullValue downgradeOnControlSplit() {
          IsNullValue value = this;
  
          if (NCP_EXTRA_BRANCH) {
              // Experimental: track two distinct kinds of "null on complex path"
              // values.
              if (value.isNullOnSomePath()) {
                  value = nullOnComplexPathValue();
              } else if (value.equals(nullOnComplexPathValue())) {
                  value = nullOnComplexPathValue3();
              }
  
          } else {
              // Downgrade "null on simple path" values to
              // "null on complex path".
              if (value.isNullOnSomePath()) {
                  value = nullOnComplexPathValue();
              }
          }
          return value;
      }
docs: Control split: move given value down in the lattice if it is a
  conditionally-null value.
  
  @return another value (equal or further down in the lattice)