RFR: 8338717: GenShen: Fix up some assertions

[Aleksey Shipilev]

On Wed, 21 Aug 2024 08:17:34 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> In https://github.com/openjdk/jdk/pull/20395 Aleksey provided some suggestions for improving various assertions. This ticket gathers together several of those changes.
> 
> **Testing:**
> - [ ] GHA
> - [ ] jtreg:tier1
> - [ ] Codepipeline
> - [ ] SPECjbb w/GenShen

Marked as reviewed by shade (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/478#pullrequestreview-2250219242