RFR: 8370822: Improve performance for MemorySegment::fill

[Per Minborg]

On Fri, 31 Oct 2025 16:10:28 GMT, Igor Rudenko <duke at openjdk.org> wrote:

>> The manual byte-replication expression replaced with the multiplicative expansion trick - a more compact and efficient equivalent.
>
> Here is syntactic example 
> 
> Before
> <img width="2413" height="902" alt="before" src="https://github.com/user-attachments/assets/3a1f83d0-be8a-48a5-a7e0-217334f01ac7" />
> 
> After
> <img width="2413" height="902" alt="after" src="https://github.com/user-attachments/assets/ea062b60-8ad1-4572-9b3b-e7ef203065ee" />

Thanks for the example @hextriclosan. I think the data you presented looks promising.

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PR Comment: https://git.openjdk.org/jdk/pull/28031#issuecomment-3479614788