RFR: Clean up CondCardMark code: remove excess barriers, enable it by default [v2]

[Zhengyu Gu]

On Wed, 31 Mar 2021 14:24:02 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> The `StoreLoad`, `StoreStore` barriers in that code are only needed for CMS, and does not relate to Shenandoah. Also, `UseCondCardMark` is a recommended option for most machines today, and therefore we better just enable it ergonomically.
>> 
>> Additional testing:
>>  - [x] hotspot_gc_shenandoah, x86_64, {fastdebug,release}
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More cleanups of scanned_concurrently

Looks goodf

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Marked as reviewed by zgu (Reviewer).

PR: https://git.openjdk.java.net/shenandoah/pull/26