RFR: 8286163: micro-optimize Instant.plusSeconds

[lennartfricke]

Provide micro-benchmark for comparison

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Commit messages:
 - 8286163: micro-optimize Instant.plusSeconds

Changes: https://git.openjdk.java.net/jdk/pull/8542/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8542&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286163
  Stats: 94 lines in 2 files changed: 93 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8542.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8542/head:pull/8542

PR: https://git.openjdk.java.net/jdk/pull/8542