RFR: 8230203: Replace markWord enums with uintptr_t constants

[Kim Barrett]

> On Aug 27, 2019, at 10:05 PM, David Holmes <david.holmes at oracle.com> wrote:
> In markWord.hpp:
> 
> 334     assert(age <= max_age, "age too large");
> 
> This assert is a tautology as age and max_age are the same uint type. (I expect some compilers may point this out.)

max_age == age_mask
        == right_n_bits(age_bits)
        == right_n_bits(4)