RFR: 8336914: Shenandoah: Missing verification steps after JDK-8255765

[Aleksey Shipilev]

On Fri, 26 Jul 2024 21:20:27 GMT, Satyen Subramaniam <duke at openjdk.org> wrote:

> Adding before-update-refs verification step which was removed in previous revision [JDK-8255765](https://bugs.openjdk.org/browse/JDK-8255765) as directed by [JDK-8336914](https://bugs.openjdk.org/browse/JDK-8336914)

Looks right.

-------------

Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20364#pullrequestreview-2210596385