RFR: 8357531: The `SegmentBulkOperations::fill` method can be improved using overlaps [v8]

[Per Minborg]

> This PR builds on a concept John Rose told me about some time ago. Instead of combining memory operations of various sizes, a single large and skewed memory operation can be made to clean up the tail of remaining bytes.
> 
> This has the effect of simplifying and shortening the code. The number of branches to evaluate is reduced.
> 
> It should be noted that the performance of the fill operation affects the allocation of new segments (as they are zeroed out before being returned to the client code).
> 
> This PR passes tier1, tier2, and tier3 on multiple platforms.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Use a fixed threashold for fill

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25383/files
  - new: https://git.openjdk.org/jdk/pull/25383/files/e9e8d15e..0da4c0be

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25383&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25383&range=06-07

  Stats: 11 lines in 2 files changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25383.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25383/head:pull/25383

PR: https://git.openjdk.org/jdk/pull/25383