RFR: 8371854: Shenandoah: Simplify WALK_FORWARD_IN_BLOCK_START use [v2]

[Aleksey Shipilev]

On Mon, 17 Nov 2025 23:31:22 GMT, Nityanand Rai <duke at openjdk.org> wrote:

>> Replace define/undefine pattern with #ifdef ASSERT block
>
> Nityanand Rai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   incorporate feedback

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28333#pullrequestreview-3475774871