RFR: 8221723: Avoid storing zero to String.hash

[Brian Goetz]

There's another reason to avoid these writes, besides CDS optimizations: 
do-nothing writes generate useless card mark activity.

On 4/1/2019 7:57 AM, Claes Redestad wrote:
> Hi,
>
> when a String has a calculated hash code value of 0, we recalculate and
> store a 0 to the String.hash field every time (except for the empty
> String, which is special cased). To make String objects more amenable to
> storage in shared read-only memory, e.g., CDS archives, we should avoid
> this redundant store.
>
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8221723
> Webrev: http://cr.openjdk.java.net/~redestad/8221723/
>
> Testing: tier1-3, no regression on existing and new StringHashCode
> micros
>
> /Claes
>