RFR: 8370822: Improve performance for MemorySegment::fill

[Per Minborg]

On Wed, 29 Oct 2025 00:01:57 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.

Marked as reviewed by pminborg (Reviewer).

It should be noted that there are existing tests covering these changes.

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

PR Review: https://git.openjdk.org/jdk/pull/28031#pullrequestreview-3410142385
PR Comment: https://git.openjdk.org/jdk/pull/28031#issuecomment-3479629459