RFR: 8260048: Shenandoah: ShenandoahMarkingContext asserts are unnecessary

[Roman Kennke]

On Wed, 20 Jan 2021 13:20:41 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> There are two `shenandoah_assert_not_forwarded` asserts that are not necessary in `mark_{strong,weak}`, because the only caller [already asserts](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoahMark.inline.hpp#L272) this higher-level invariant. There is no need to check it in `ShenandoahMarkingContext` once again. This simplifies the fastpath in fastdebug builds.
> 
> Additional testing:
>  - [x] `hotspot_gc_shenandoah`
>  - [x] `tier1`, `tier2` with Shenandoah

Ok.

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2164