JavaValueFilter mergeFilter(PsiElement expression) {
    DfaBasedFilter dfaFilter = myDfaFilter == null ? new DfaBasedFilter(DfType.TOP) : myDfaFilter;
    DfaBasedFilter newFilter = dfaFilter.mergeFilter(expression);
    return dfaFilter == newFilter ? this : new JavaValueFilter(newFilter, myStackFilter);
  }
JavaValueFilter mergeFilter(PsiElement expression)