[foreign-memaccess] RFR: 8248487: Add static helpers to access segments (continued) - benchmark fix

[Maurizio Cimadamore]

This patch fixes the benchmarks that were referring to the previous accessor names.

It also addresses a performance loophole with long multiplication being used even when index fits in an int.

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Commit messages:
 - Fix benchmarks

Changes: https://git.openjdk.java.net/panama-foreign/pull/233/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/233/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248487
  Stats: 52 lines in 4 files changed: 11 ins; 0 del; 41 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/233.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/233/head:pull/233

PR: https://git.openjdk.java.net/panama-foreign/pull/233