RFR: 8331252: C2: MergeStores: handle negative shift values

[Aleksey Shipilev]

On Mon, 29 Apr 2024 15:43:06 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> Somehow, I have not thought of negative shift constants, and there was no regression test for it.
> The fuzzer now found a case.
> 
> **I convert the assert into a condition.**

Ah, just caught it in my testing. Fix looks good.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19001#pullrequestreview-2029137160